Nanowire structures comprising carbon

ABSTRACT

The present invention is directed to nanowire structures and interconnected nanowire networks comprising such structures, as well as methods for their production. The nanowire structures comprise a nanowire core, a carbon-based layer, and in additional embodiments, carbon-based structures such as nanographitic plates consisting of graphenes formed on the nanowire cores, interconnecting the nanowire structures in the networks. The networks are porous structures that can be formed into membranes or particles. The nanowire structures and the networks formed using them are useful in catalyst and electrode applications, including fuel cells, as well as field emission devices, support substrates and chromatographic applications.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 11/295,133, filed Dec. 6, 2005, now U.S. Pat. No.7,179,561, which claims the benefit of U.S. Provisional PatentApplication Nos. 60/634,472, filed Dec. 9, 2004, and 60/738,100, filedNov. 21, 2005. The present application also claims the benefit of U.S.Provisional Patent Application No. 60/801,377, filed May 19, 2006, andU.S. Provisional Patent Application No. 60/738,100, filed Nov. 21, 2005.The disclosures of each of these applications are incorporated herein byreference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to various nanowire structures, and tointerconnected nanowire networks comprising such structures. The presentinvention also relates to the use of these nanowire structures andinterconnected nanowire networks in fuel cells, as field emittingelements and other applications, such as chromatographic materials.

2. Background of the Invention

Nanomaterials, and in particular, nanowires have the potential tofacilitate a whole new generation of electronic devices. For example, Incertain cases, uses of nanomaterials have been proposed that exploit theunique and interesting properties of these materials more as a bulkmaterial than as individual elements requiring individual assembly. Forexample, Duan et al., Nature 425:274-278 (September 2003), describes ananowire based transistor for use in large area electronic substrates,such as, displays, antennas, and the like that employ a bulk processed,oriented semiconductor nanowire film or layer in place of a rigidsemiconductor wafer. The result is an electronic substrate that performson par with a single crystal wafer substrate that can be manufacturedusing conventional and less expensive processes than those used tomanufacture poorer performing amorphous semiconductors, which is alsomore amenable to varied architectures, such as, flexible and/or shapedmaterials.

Graphene layers, a single atom layer of carbon formed as a sheet, havefound applications in electrochemical cells and as flexible, strongsubstrates and coatings. See e.g., U.S. Pat. Nos. 5,677,082, 6,303,266and 6,479,030. In addition, rolled or folded layers of graphene can formcarbon nanotubes and the like, see e.g., U.S. Pat. Nos. 6,582,673,6,749,827 and 6,756,026.

Fuel cells are devices that convert the chemical energy of fuels, suchas hydrogen and methanol, directly into electrical energy. The basicphysical structure or building block of a fuel cell consists of anelectrolyte layer in contact with a porous anode and cathode on eitherside. A schematic representation of a fuel cell with thereactant/product gases and the ion conduction flow directions throughthe cell is shown in FIG. 6. In a typical fuel cell as shown in FIG. 6,a fuel (e.g., methanol or hydrogen) is fed to an anode catalyst thatconverts the fuel molecules into protons (and carbon dioxide formethanol fuel cells), which pass through the proton exchange membrane tothe cathode side of the cell. At the cathode catalyst, the protons(e.g., hydrogen atoms without an electron) react with the oxygen ions toform water. By connecting a conductive wire from the anode to thecathode side, the electrons stripped from fuel, hydrogen or methanol onthe anode side, can travel to the cathode side and combine with oxygento form oxygen ions, thus producing electricity. Fuel cells operating byelectrochemical oxidation of hydrogen or methanol fuels at the anode andreduction of oxygen at the cathode are attractive power sources becauseof their high conversion efficiencies, low pollution, lightweight, andhigh energy density.

For example, in direct methanol fuel cells (DMFCs), the liquid methanol(CH₃OH) is oxidized in the presence of water at the anode generatingCO₂, hydrogen ions and the electrons that travel through the externalcircuit as the electric output of the fuel cell. The hydrogen ionstravel through the electrolyte and react with oxygen from the air andthe electrons from the external circuit to form water at the anodecompleting the circuit.Anode Reaction: CH₃OH+H₂O→CO₂+6H++6e−Cathode Reaction: 3/2 O₂+6H++6e−→3H₂OOverall Cell Reaction: CH₃OH+3/2 O₂→CO₂+2H₂O

Initially developed in the early 1990s, DMFCs were not embraced becauseof their low efficiency and power density, as well as other problems.Improvements in catalysts and other recent developments have increasedpower density 20-fold and the efficiency may eventually reach 40%. Thesecells have been tested in a temperature range from about 50° C.-120° C.This low operating temperature and no requirement for a fuel reformermake the DMFC an excellent candidate for very small to mid-sizedapplications, such as cellular phones, laptops, cameras and otherconsumer products, up to automobile power plants. One of the drawbacksof the DMFC is that the low-temperature oxidation of methanol tohydrogen ions and carbon dioxide requires a more active catalyst, whichtypically means a larger quantity of expensive platinum (and/orruthenium) catalyst is required.

A DMFC typically requires the use of ruthenium (Ru) as a catalystcomponent because of its high carbon monoxide (CO) tolerance andreactivity. Ru disassociates water to create an oxygenated species thatfacilitates the oxygenation of CO, which is produced from the methanol,to CO₂. Some existing DMFCs use nanometer-sized bimetallic Pt:Ruparticles as the electro-oxidation catalyst because of the high surfacearea to volume ratio of the particles. The Pt/Ru nanoparticles aretypically provided on a carbon support (e.g., carbon black, fullerenesoot, or desulfurized carbon black) to yield a packed particle compositecatalyst structure. Most commonly used techniques for creating the Pt:Rucarbon packed particle composite are the impregnation of a carbonsupport in a solution containing platinum and ruthenium chloridesfollowed by thermal reduction

A multi-phase interface or contact is established among the fuel cellreactants, electrolyte, active Pt:Ru nanoparticles, and carbon supportin the region of the porous electrode. The nature of this interfaceplays a critical role in the electrochemical performance of the fuelcell. It is known that only a portion of catalyst particle sites inpacked particle composites are utilized because other sites are eithernot accessible to the reactants, or not connected to the carbon supportnetwork (electron path) and/or electrolyte (proton path). In fact,current packed particle composites only utilize about 20 to 30% of thecatalyst particles. Thus, most DMFCs which utilize packed particlecomposite structures are highly inefficient.

In addition, connectivity to the anode and/or cathode is currentlylimited in current packed particle composite structures due to poorcontacts between particles and/or tortuous diffusion paths for fuel cellreactants between densely packed particles. Increasing the density ofthe electrolyte or support matrix increases connectivity, but alsodecreases methanol diffusion to the catalytic site. Thus, a delicatebalance must be maintained among the electrode, electrolyte, and gaseousphases in the porous electrode structure in order to maximize theefficiency of fuel cell operation at a reasonable cost. Much of therecent effort in the development of fuel cell technology has beendevoted to reducing the thickness of cell components while refining andimproving the electrode structure and the electrolyte phase, with theaim of obtaining a higher and more stable electrochemical performancewhile lowering cost. In order to develop commercially viable DMFCs, theelectrocatalytic activity of the catalyst must be improved.

A structure combining nanowires, for example semiconductor nanowires,and graphene layers has not been heretofore been disclosed. In addition,nanowires comprised of carbon, or covered with a carbon-based layer,have also not heretofore been disclosed. The nanowire-based structuresand networks disclosed herein possess unique properties andcharacteristics that allow their use in various applications fromsubstrates and supports to membranes and filtration. The presentinvention also provides nanowire composite membrane electrode catalystsupport assemblies comprising the various structures describedthroughout that provide a highly porous material with a high surfacearea, a high structural stability and a continuum structure. Thecomposite structures are provided as a highly interconnected nanowiresupported catalyst structure interpenetrated with an electrolyte networkto maximize catalyst utilization, catalyst accessibility, and electricaland ionic connectivity to thereby improve the overall efficiency of fuelcells, at lower cost, etc.

SUMMARY OF THE INVENTION

The present invention provides nanowire structures and interconnectednanowire networks. These structures and networks are particularly usefulas membranes and supports in various catalyst and battery applications,as high surface area electrodes in medical devices, as well as particlesfor use in chromatography.

In an embodiment, the present invention provides nanowires comprising acarbon-based layer. Suitably the carbon-based layer is non-crystallineand substantially devoid of basal plane carbon. In an embodiment, thenanowires comprise a core which can comprise semiconductor material suchas Si and B. In additional embodiments, the core can comprise carbon,such as a carbide (e.g., SiC), or can consist only of carbon (i.e., apure carbon nanowire). In other embodiments, the carbon-based layer iscarbide, for example SiC. Generally, the carbon-based layer on thenanowires will be greater than about 1 nm in thickness, though thinnerlayers can also be prepared.

In another embodiment, the present invention provides nanowirestructures, comprising a core, an interfacial carbide layer and acarbon-based structure formed on the interfacial carbide layer. Examplesof carbon-based structures that can be formed on the interfacial carbidelayer include nanowires and nanographitic plates. Suitably, the nanowirecore will be made from semiconductor material, such as Si, B or SiC,and/or a highly doped semiconductor material. Additional materials thatcan be used to form the core include, but are not limited to, inorganicoxides, inorganic carbides and inorganic nitrides, as well as carbonnanotubes and carbon nanofibrils. Nanographitic plates suitably comprisemultiple layers of graphene attached to the interfacial carbide layer,and to each other.

The present invention also provides methods of manufacturing suchnanowire structures. An embodiment comprises: heating a nanowire coreand contacting the nanowire core with one or more carbon-comprisinggases to form a carbon-based layer on the nanowire core. Generally, thetemperatures at which the carbon-based layers are formed are greaterthan about 600° C. Suitable carbon-comprising gases that can be used inthe methods of manufacturing include, but are not limited to, carbonmonoxide, methane, ethane, propane, butane, ethylene and propylene. Inaddition, the gas mixtures can also comprise noble or othernon-contaminating gases. In other embodiments, hydrogen can be includedto control the carbide layer and nanographite formation process.

The present invention also comprises methods of manufacturing nanowirestructures comprising: heating a nanowire core; contacting the nanowirecore with one or more carbon-comprising gases to form an interfacialcarbide layer on the nanowire core; and forming at least onecarbon-based structure on the interfacial carbide layer. Generally, thetemperatures at which the interfacial carbide layers and carbon-basedstructures (e.g., nanowires and/or nanographitic plates) are formed aregreater than about 600° C. Suitable carbon-comprising gases that can beused in the methods of manufacturing include, but are not limited to,methane, ethane, propane, butane, ethylene and propylene. In addition,the gas mixtures can also comprise noble or other non-contaminatinggases. In other embodiments, hydrogen can be included to control thecarbide layer and carbon-based structure formation process.

Methods of manufacturing the interconnected nanowire networks are alsoprovided. An exemplary method comprises: dispersing a plurality ofnanowire cores in a liquid; filtering the nanowire cores to form ananowire mat; heating the nanowire mat; contacting the nanowire mat withone or more carbon-comprising gases to form an interfacial carbide layeron the nanowire cores, and forming nanographitic plates on theinterfacial carbide layer, such that that nanographitic platesinterconnect the nanowire cores.

In another embodiment, the present invention provides interconnectednanowire networks, comprising, for example, a plurality of nanowirestructures, wherein nanographitic plates connect the nanowirestructures. The nanowire networks suitably form mesoporous membranes orparticles. The nanowire structures and interconnected nanowire networkshave applications in areas such as catalysts, fuel cells, high surfacearea electrodes for medical devices, sensors, support substrates andmedia for chromatography and filtration.

In another embodiment, the present invention provides a proton exchangemembrane fuel cell with nanostructured components, in particular, one ormore of the electrodes of the membrane electrode assembly. Thenanostructured fuel cell has a higher catalytic metal utilization rateat the electrodes, higher power density (kW/volume and kW/mass), andlower cost than conventional fuel cells. The nanostructured fuel cellsare not only attractive for stationary and mobile applications, but alsofor use as a compact power supply for microelectronics such as laptops,cell phones, cameras and other electronic devices.

In accordance with a further embodiment of the present invention,nanowires (e.g., inorganic nanowires) for use in a membrane electrodeassembly of a fuel cell are disclosed which generally comprise a metalcatalyst deposited on a surface of the nanowires. The metal catalyst maybe deposited as a thin film on the surface of the nanowires, or as alayer of catalyst particles, e.g., by functionalizing the surface of thenanowires with standard surface chemistries. Suitable metal catalystsinclude, but are not limited to, one or more of platinum (Pt), ruthenium(Ru), iron (Fe), cobalt (Co), gold (Au), chromium (Cr), molybdenum (Mo),tungsten (W), manganese (Mn), technetium (Tc), rhenium (Re), osmium(Os), rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), copper(Cu), silver (Ag), zinc (Zn), tin (Sn), aluminum (Al), and combinationsand alloys thereof (such as bimetallic Pt:Ru nanoparticles). Thenanowires may comprise branched structures (e.g., side nodules) toincrease the surface area to volume ratio of the wires to maximize thecatalytic efficiency of the fuel cell. The nanowires may be made frommetallic conducting, semiconducting, carbide, nitride, or oxidematerials such as RuO₂, SiC, GaN, TiO₂, SnO₂, WCx, MoCx, ZrC, WN_(x),MoN_(x) etc. It is preferable that the nanowires be made from a materialthat is resistant to degradation in a weak acid so that the nanowiresare compatible with the reactants of a variety of different fuel cells.

The nanowires of the present invention may be derivatized with at leasta first functional group or chemical binding moiety which binds tometallic catalyst particles, such as a nitric acid group, carboxylicacid group, a hydroxyl group, an amine group, a sulfonic acid group, andthe like, or the catalyst may be deposited as a thin film using otherdeposition processes such as electrodeposition, atomic layer deposition,plasma sputtering, etc. The nanowires may also be derivatized with afunctional group which differentially binds to a thin proton conductingpolymer coating (e.g., NAFION® or other sulfonated polymer) which may bedeposited directly on the nanowires. For example, the nanowires may befunctionalized with a sulfonated hydrocarbon, fluorocarbon, or branchedhydrocarbon chain using known standard chemistries. Alternatively,instead of binding ionomer to the nanowires through a chemical bindingmoiety, the nanowires may be functionalized to make them protonconductive. For example, the nanowires may be functionalized with asurface coating such as a perfluorinated sulfonated hydrocarbon usingwell-known functionalization chemistries.

In this way, the intimate relationship between the nanowire catalystsupport and the polymer shell ensures that most, if not all, of themetal catalyst particles are located at a three-phase contact point(e.g., such that the catalyst particles are accessible to the fuel cellreactants, electrolyte and nanowire core for efficient electron andproton conduction). The controlled nanowire surface chemistry can beused to control the wettability of the polymer in the composite nanowirestructure and ensures that catalyst particles are exposed and accessiblefor catalysis.

According to another embodiment of the present invention, ananostructured catalyst support for a membrane electrode assembly of afuel cell is provided which generally comprises an interconnected mat ornetwork of nanowires each having a metal catalyst deposited thereon. Thecatalyst metal may comprise any of the catalyst metals described herein,as well as those known to the ordinarily skilled artisan, such asplatinum. The catalyst metal may comprise a combination of metals suchas platinum and ruthenium. In one representative embodiment, thecatalyst metal comprises nanoparticles having a diameter less than about50 nm, e.g., less than about 10 nm, e.g., less than about 5 nm, e.g.,between about 1 and 5 nm. In this embodiment, each nanowire in thenetwork of nanowires typically is physically and/or electricallyconnected to at least one or more other nanowires in the nanowirenetwork to form a highly interconnected network of nanowires. In otherembodiments, the nanowires may be substantially aligned in a parallelarray of nanowires between the anode/cathode bipolar plates and theproton exchange membrane, or the nanowires may be randomly oriented. Thenanowires may each be coated with a first catalyst colloid coatingand/or a second thin proton conducting polymer coating (e.g., NAFION®).The membrane electrode assembly may be a component in a direct methanolfuel cell, a hydrogen fuel cell, or any other fuel cell known to thoseof ordinary skill in the art.

A fuel cell is formed by providing a proton exchange membrane, an anodeelectrode, a cathode electrode, and first and second bipolar plates,wherein at least one of the anode and cathode electrode comprise aninterconnected network of the catalyst supported nanowires. Because ofthe superior connectivity of the nanowire network, the fuel cell may notrequire a gas diffusion layer between the proton exchange membrane andthe first or second bipolar plates as is the case with conventional fuelcells. In one embodiment, the nanowires may be synthesized directly onone or more of the bipolar plates of the fuel cell and/or on the protonexchange membrane. The nanowires may also be grown on a separate growthsubstrate, harvested therefrom, and then transferred (e.g., as a poroussheet of interconnected wires) and incorporated into the fuel cellstructure (e.g., deposited on one or more of the fuel cell componentssuch as one or more of the bipolar plates and/or the proton exchangemembrane). When grown in situ on the bipolar plate(s) and/or protonexchange membrane, the nanowires may be oriented substantiallyperpendicular or normal to a surface of the bipolar plate(s) or protonexchange membrane, or oriented randomly.

The nanowires in the nanowire network are preferentially physicallyand/or electrically connected to one or more other wires in the networkto form an open, highly branched, porous, intertwined structure, withlow overall diffusion resistance for reactants and waste diffusion, highstructural stability and high electrical connectivity for the electronsto ensure high catalytic efficiency, thus leading to high power densityand lower overall cost. The multiple electrical connectivity of thenanowires ensures that if one wire breaks or is damaged in the system,for example, that all points along the wire still connect to the anode(or cathode) electrode along different paths (e.g., via other nanowiresin the network). This provides substantially improved electricalconnectivity and stability as compared to previous packed particlecomposite structures. The catalyst is highly accessible to the fuelsource to produce electrons and protons, while the electrons can conductdirectly to the bipolar plate through the nanowire and the protons cantransport directly to the membrane through the polymer.

The nanowires in the network of nanowires may be cross-linked or fusedtogether using various cross-linking or sintering methods describedfurther herein at points where such nanowires contact or are proximal toothers of the nanowires to increase the connectivity and structuralstability of the nanowire network. In another embodiment, the samestrategy of cross-linking or sintering can be used to improve theelectrical or structural connectivity between the nanowires and catalystmaterial that is in contact or proximal with such nanowires.

The nanowire network defines a plurality of pores between the nanowiresin the network, wherein the plurality of pores preferentially have aneffective pore size of less than about 10 μm, for example, less thanabout 5 μm, e.g., less than about 1 μm, e.g., less than about 0.2 μm,e.g., less than 0.02 μm, e.g., between about 0.002 μm and 0.02 μm, e.g.,between about 0.005 and 0.01 μm. The overall porosity of the branchednanowire structure may be greater than about 30%, for example, betweenabout 30% and 95%, e.g., between about 40% and 60%. The nanowires aredispersed in a porous polymer matrix electrolyte material such asperfluorosulfonic acid/PTFE copolymer (e.g., NAFION®) which forms acontinuous network interpenetrated with the nanowires in the branchednanowire network to provide sufficient contact points for proton (e.g.,H+) transport.

In another embodiment of the present invention, a method for preparing afuel cell membrane electrode is disclosed which generally comprises (a)associating a catalyst metal selected from the group comprising one ormore of chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn),technetium (Tc), rhenium (Re), iron (Fe), ruthenium (Ru), osmium (Os),cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd),platinum (Pt), copper (Cu), silver (Ag), gold (Au), zinc (Zn), tin (Sn),aluminum (Al), and combinations thereof, with a plurality of inorganicnanowires to form a plurality of inorganic nanowires with associatedcatalyst metal, and (b) forming a membrane electrode comprising aplurality of inorganic nanowires with associated catalyst metal.

The plurality of inorganic nanowires may be derivatized with at least afirst functional group which binds the catalyst metal such as a nitricacid group, a carboxylic acid group, a hydroxyl group, an amine group, asulfonic acid group, and the like. The associating may also be done by avariety of methods selected from the group comprising chemical vapordeposition, electrochemical deposition, physical vapor deposition,solution impregnation and precipitation, colloid particle absorption anddeposition, atomic layer deposition, and combinations thereof. Forexample, the associating may be done by chemical deposition of acatalyst metal precursor such as chloroplatinic acid or byelectrodeposition of Pt from a precursor salt in solution. The catalystmetal precursor may be converted to a catalytically active metal bysubjecting the catalyst metal precursor to metal reduction, whereinmetal reduction is done by a method selected from the group comprisinghydrogen reduction, chemical reduction, electrochemical reduction and acombination thereof. The catalytically active metal may be in the formof metal nanoparticles on the surface of the nanowires. The forming maybe done on a proton exchange membrane or on one or more of the bipolarplates, for example, by a method selected from the group comprisingspray/brush painting, solution coating, casting, electrolyticdeposition, filtering a fluid suspension of the nanowires, andcombinations thereof. The nanowires may also be grown directly on one ormore of the fuel cell components such as one or more of the bipolarplates and/or proton exchange membrane. The method may further comprisemixing an ionomeric resin (e.g., perfluorosulfonic acid/PTFE copolymer,e.g., NAFION®) with the plurality of inorganic nanowires with associatedcatalyst metal. The plurality of inorganic nanowires may be derivatizedwith at least a second functional group (e.g., a sulfonated hydrocarbongroup) which binds the ionomeric resin.

In another embodiment of the present invention, a method of making amembrane electrode assembly of a fuel cell is disclosed which generallycomprises: forming nanowires on a growth substrate; transferring thenanowires from the growth substrate into a fluid suspension; depositingone or more catalyst metals on the nanowires to form a nanowiresupported catalyst; filtering the fluid suspension of nanowires tocreate a porous sheet of interconnected nanowires; infiltrating thenetwork of nanowires with an ionomeric resin; and combining the sheet ofinterconnected nanowires with a proton exchange membrane to form amembrane electrode assembly (MEA). Hot pressing may be used to fuseelectrolyte in both the anode and cathode electrode with the protonexchange membrane to form a continuous electrolyte phase for efficientproton transport from the anode electrode to the cathode electrode. Thestep of depositing one or more catalyst metals may comprise, forexample, depositing a metal selected from the group comprising platinum,gold, ruthenium, and other metals, and combinations thereof. The methodmay further comprise forming a proton exchange membrane fuel cellutilizing the formed MEA by combining first and second bipolar platestogether to form the proton exchange membrane fuel cell.

In another embodiment, the present invention provides fuel cellelectrodes comprising an inorganic support wafer having a first surfacewith one or more channels, one or more nanowires disposed within thechannels, and one or more metal catalysts deposited on a surface of theone or more nanowires. In suitable embodiments, the inorganic supportwafer is a Si wafer that is on the order of about 5 mm or less inthickness. In additional embodiments, the wafer has a second surfacecomprising one or more channels opposite the first surface, one or morenanowires disposed within the channels in the second surface, and one ormore metal catalysts deposited on a surface of the nanowires. Suitablythe surfaces of the wafer and the nanowires will be carburized. Examplesof metal catalysts useful in the fuel cell electrodes include thosedescribed throughout, including Pt, Au, Pd, Ru, Re, Rh, Os, Ir, Fe, Co,Ni, Cu, Ag, V, Cr, Mo, W and alloys and mixtures thereof. Suitably, themetal catalysts will be nanoparticles having a diameter of between about1 nm and 10 nm.

The present invention also provides membrane electrode assembliescomprising a first fuel cell electrode, a proton exchange membrane, anda second fuel cell electrode. In suitable embodiments, the first surfaceof the first fuel cell electrode comprises nanowires with anionicmetallic catalysts (e.g., PtRu) and the second surface of the secondfuel cell electrode comprises nanowires with cationic metallic catalysts(e.g., Pt). A suitable proton exchange membrane for use in the presentinvention includes a sulfonated tetrafluorethylene copolymer.

The present invention also provides methods for preparing a fuel cellelectrode comprising providing a semiconductor wafer having a firstsurface and a second surface, etching one or more channels on the firstsurface and the second surface, disposing one or more nanowires in thechannels in the first and second surfaces, contacting the nanowires andthe first and second surfaces with one or more carbon-comprising gasesto form a carbon-based layer on the nanowires and the first and secondsurfaces, and depositing one or more metal catalysts on the nanowires.Suitable etching methods include NaOH etching. Generally, nanoparticlecatalysts will be deposited on the nanowires to generate cathode andanode nanowires.

In a further embodiment, the present invention provides field emissionelements comprising a nanowire structure with a core, an interfacialcarbide layer and a carbon-based structure formed on the interfacialcarbide layer. Suitably, the core comprises semiconductor material, suchas Si, B, SiC or GaN. In additional embodiments, the core comprises aninorganic oxide selected from the group comprising SiO₂, Al₂O₃, TiO₂,SnO₂, ZrO₂, HfO₂ and Ta₂O₅; an inorganic carbide selected from the groupcomprising TiC, ZrC, HfC, NbC, WC, W₂C, MoC and Mo₂C; or an inorganicnitride selected from the group comprising TiN, ZrN, HfN, WN, MoN andBN. The interfacial carbide layer suitably is selected from the groupcomprising SiC, TiC, ZrC, HfC, NbC, WC, Mo₂C and mixtures thereof. Thecarbon-based structure generally is at least one nanographitic plateextending away from the core a distance of about 1 nm to about 100 nmand is oriented relative the major axis of the core at an angle ofbetween about 0° and about 90°.

In additional embodiments, the present invention provides methods forpreparing one or more nanowires comprising one or more catalyst metalsassociated with the nanowires. In exemplary embodiments, one or morenanowires are dispersed in a solution, and then one or more catalystmetals are added (for example, a solution comprising one or morecatalyst metal nanoparticles). The solution is then refluxed (e.g.,heated until boiling, for example, for about 10-60 minutes), whereby thecatalyst metals become associated with the nanowires. The solution canthen be filtered to generate a solid nanowire dispersion, and thenfinally dried.

The present invention also provides methods for preparing a fuel cellmembrane electrode assembly. Initially, a gas diffusion layer isprovided. Then, a first composition (e.g., a solution) of catalystmetal-associated nanowires is disposed adjacent the gas diffusion layer.Suitably, the solution of catalyst metal-associated nanowires alsocomprises one or more ionomers) Then, a membrane layer (e.g., a protonconducting polymer) is disposed adjacent the first catalystmetal-associated nanowire composition, and finally, a second compositionof catalyst metal-associated nanowires (e.g., a solution suitably alsocomprising one or more ionomers) is disposed adjacent the membranelayer. Suitably the first composition comprises anode catalystmetal-associated nanowires and the second composition comprises cathodecatalyst metal-associated nanowires. In additional embodiments, amasking layer is disposed adjacent the gas diffusion layer to cover atleast the edges of the gas diffusion layer prior to disposing the firstcomposition of catalyst metal-associated nanowires. The masking layer isthen removed after the disposing of the first composition, but prior todisposing the membrane layer. In further embodiments, a masking layer isdisposed on the membrane layer to cover at least the edges of themembrane layer prior to disposing the second composition of catalystmetal-associated nanowires. Suitably the nanowire compositions aresprayed from solution. The present invention also provides membraneelectrode assemblies prepared according to the methods disclosedthroughout.

In a still further embodiment, the present invention provides methodsfor preparing a fuel cell electrode stack. Initially, a first end plateis provided, and a gasket is disposed adjacent the end plate. Then, amembrane electrode assembly (MEA) of the present invention is disposedadjacent the gasket. A gas diffusion layer is then disposed adjacent theMEA, followed by disposing another gasket adjacent the gas diffusionlayer. Finally, a end plate is disposed adjacent the second gasket.

In further embodiments, the MEA preparation methods of the presentinvention can further comprise assembling additional MEA layers (2, 3,4, 5, 6, etc., up to an n^(th) MEA) when preparing fuel cell electrodestacks. For example, following disposition of the second gasket, andprior to disposing the final end plate, a bipolar plate is disposedadjacent the second gasket. Another gasket is then disposed adjacent thebipolar plate, and a second MEA is disposed adjacent the gasket. This isfollowed by disposing an additional gas diffusion layer adjacent theMEA. Finally, an additional gasket is then disposed the adjacent gasdiffusion layer. These steps can then repeated until the n^(th), i.e.,final, membrane electrode assembly, has been disposed, prior todisposing an end plate.

The present invention also provides conducting composites, comprisingone or more nanowires comprising a core, an interfacial carbide layerand a carbon-based structure formed on the interfacial carbide layer,and carbon black. An additional embodiment provides porous catalystsupports comprising one or more nanowires comprising a core, aninterfacial carbide layer and a carbon-based structure formed on theinterfacial carbide layer, and carbon black. A still further embodimentprovides catalysts comprising one or more nanowires comprising a core,an interfacial carbide layer and a carbon-based structure formed on theinterfacial carbide layer, carbon black and one or more catalyst metalsassociated with the nanowires and the carbon black.

Further embodiments, features, and advantages of the invention, as wellas the structure and operation of the various embodiments of theinvention are described in detail below with reference to accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described with reference to the accompanying drawings.In the drawings, like reference numbers indicate identical orfunctionally similar elements. The drawing in which an element firstappears is indicated by the left-most digit in the correspondingreference number.

FIG. 1A is a transmission electron micrograph of a nanowire structure inaccordance with one embodiment of the present invention.

FIG. 1B is a high magnification transmission electron micrograph of ananowire structure in accordance with one embodiment of the presentinvention.

FIG. 2 is an x-ray diffraction pattern of a nanowire structure inaccordance with one embodiment of the present invention.

FIG. 3A is a scanning electron micrograph of an interconnected nanowirenetwork in accordance with one embodiment of the present invention.

FIG. 3B is a high magnification scanning electron micrograph of aninterconnected nanowire network in accordance with one embodiment of thepresent invention.

FIG. 4 shows a flow chart representing a method of manufacturing ananowire structure in accordance with one embodiment of the presentinvention.

FIG. 5 shows a flow chart representing a method of manufacturing aninterconnected nanowire network in accordance with one embodiment of thepresent invention.

FIG. 6 is a schematic representation of a conventional electrochemicalfuel cell showing exemplary reactions in the anode and the cathodeelectrodes.

FIG. 7A is an expanded view of the anode electrode portion of the fuelcell of FIG. 6 showing details of a conventional packed particlecomposite catalyst structure comprising Pt/Ru nanoparticles provided ona carbon particle support.

FIG. 7B is an expanded view of the packed particle composite catalyststructure of FIG. 7A showing an exemplary three-phase contact betweenthe gaseous reactants, electrolyte, and the electrocatalyst structure.

FIG. 8A is a schematic representation of a nanowire-basedelectrochemical fuel cell made according to the teachings of the presentinvention.

FIG. 8B is a schematic representation of a nanowire-basedelectrochemical fuel cell stack made according to the teachings of thepresent invention.

FIG. 9A is an expanded view of the anode electrode portion of the fuelcell of FIG. 8A showing details of an embodiment of an interconnectednetwork of catalyst supported nanowires which span the junction betweenthe proton exchange membrane and anode electrode of the fuel cell ofFIG. 8A.

FIG. 9B is an expanded view of an alternative embodiment for ananowire-based anode portion of a fuel cell showing details of aparallel array of catalyst supported nanowires which span the junctionbetween the proton exchange membrane and the anode electrode of the fuelcell of FIG. 8A.

FIG. 10 is a SEM image of an interconnected network of nanowires used asthe catalyst support in an anode (and/or cathode) electrode of a fuelcell made according to the teachings of the present invention.

FIG. 11 is a schematic representation of a branched nanowire structurethat can be used in practicing the methods of the present invention.

FIG. 12 is an SEM image of a branched nanowire network including aplurality of branched nanowires having tiny nodules extending from theside surfaces of the nanowires.

FIG. 13 is an SEM image at high magnification of cross-linked or fusednanowires creating an interconnecting nanowire network as used incertain aspects of the present invention.

FIG. 14 is a SEM image showing Au catalyst particles deposited on anetwork of interconnected nanowires.

FIG. 15A shows a TEM of Pt nanoparticles, average diameter 1.61±0.31 nm,in accordance with one embodiment of the present invention. FIG. 15 Bshows an X-ray diffraction pattern of these Pt nanoparticles.

FIG. 16 A shows a TEM of Pt—Ru alloy nanoparticles, 1.66±0.33 nm, inaccordance with one embodiment of the present invention. FIG. 16 B showsan X-ray diffraction pattern of these Pt—Ru alloy nanoparticles.

FIG. 17 shows TEM images of Pt—Ru alloy nanoparticles supported on thesurface of carbon-black Cabot VULCAN® XC72 at two differentmagnifications.

FIG. 18 shows X-ray diffraction patterns recorded from carbon-supportedPt—Ru catalyst of the present invention (top curve) and a commerciallyavailable Pt—Ru catalyst (bottom curve).

FIG. 19 shows a TEM image of Pt nanoparticles deposited on a nanowire inaccordance with one embodiment of the present invention.

FIG. 20 shows TEM images of 1.67 nm Pt—Ru (1:1) nanoparticles depositedon the surface of nanographite coated nanowires at two differentmagnifications.

FIG. 21 shows a method for preparing catalyst metal-associated nanowiresusing reflux in accordance with one embodiment of the present invention.

FIG. 22 shows a process for producing micro fuel cells using inorganicwafers in accordance with one embodiment of the present invention.

FIG. 23 shows a method for preparing a fuel cell membrane electrodeassembly in accordance with one embodiment of the present invention.

FIG. 24 shows an exemplary four-layer membrane electrode assembly inaccordance with one embodiment of the present invention.

FIG. 25 shows an exemplary fuel cell electrode stack in accordance withone embodiment of the present invention.

FIG. 26A shows a method for preparing fuel cell electrode stacks inaccordance with one embodiment of the present invention.

FIG. 26B shows additional methods for preparing fuel cell electrodestacks in accordance with one embodiment of the present invention.

FIG. 27 shows a TEM of graphite-coated nanowires for use as emittingelements in accordance with one embodiment of the present invention.

FIG. 28 shows a plot of current versus potential comparing the oxygenreduction activity of three catalyst electrodes.

FIG. 29 shows a plot of current versus potential comparing the methanoloxidation activity of commercially available carbon black catalyst andcatalyst material prepared according to one embodiment of the presentinvention.

FIG. 30 shows a Tafel plot for oxygen reduction on a bird's nestcatalyst prepared in accordance with the present invention, commerciallyavailable carbon black catalyst, and carbon-based catalyst prepared inaccordance with the present invention.

FIG. 31 shows methanol oxidation activity of the bird's nest catalyststructure of the present invention in comparison to commerciallyavailable carbon black catalyst and carbon-based catalyst prepared inaccordance with the present invention.

FIG. 32 shows methanol oxidation activity of the catalyst material ofthe present invention on a bird's nest support and a carbon support, aswell as commercially available carbon black catalyst.

FIG. 33 shows a plot of current versus potential, representing theoxygen reduction activity of a commercially available Pt catalyst(Electrochem Carbon) and Pt catalysts of the present invention on acarbon support and a nanowire support, in accordance with one embodimentof the present invention.

FIG. 34A shows IR corrected potential versus current densityrepresenting fuel cell performance of a commercially available membraneelectrode assembly (ECCMEA) and two nanowire-supported membraneelectrode assemblies in accordance with one embodiment of the presentinvention. FIG. 34B shows accessibility loss, with fitting at 30 mA/cm²for the same assemblies.

FIG. 35 shows IR corrected potential versus current density comparingthe effects of oxygen partial pressure on performance of platinumcatalysts of the present invention on carbon supports and nanowiresupports.

FIG. 36 shows potential variation with current density showing theeffects of oxygen pressure variation on performance of platinumcatalysts of the present invention on carbon supports and nanowiresupports.

FIG. 37 shows the results of fuel cell performance characteristics,showing IR corrected potential versus current density, comparingcommercially available Pt catalysts and Pt catalysts of the presentinvention on carbon supports and nanowire supports.

FIG. 38 shows catalyst-associated nanowires prepared by the refluxmethods of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

It should be appreciated that the particular implementations shown anddescribed herein are examples of the invention and are not intended tootherwise limit the scope of the present invention in any way. Indeed,for the sake of brevity, conventional electronics, manufacturing,semiconductor devices, and nanowire (NW), nanorod, nanotube, andnanoribbon technologies and other functional aspects of the systems (andcomponents of the individual operating components of the systems) maynot be described in detail herein. Furthermore, for purposes of brevity,the invention is frequently described herein as pertaining to nanowires,though other similar structures are also encompassed herein.

It should be appreciated that although nanowires are frequently referredto, the techniques described herein are also applicable to othernanostructures, such as nanorods, nanotubes, nanotetrapods, nanoribbonsand/or combinations thereof. It should further be appreciated that themanufacturing techniques described herein could be used to create acarbon-based layer (including non-crystalline carbon, such as non-basalplane carbon, as well as crystalline nanographite coatings) on thesurface of a wide range of materials, including, but not limited to,conventional fibers and fiber structures; flat, curved and irregularsurfaces; and various materials such as metal, semiconductors, ceramicfoams, reticulated metals and ceramics. Further, the techniques would besuitable for application as catalysts, energy storage and conversion,separation, electrodes for medical devices, protective surfaces, or anyother application.

As used herein, an “aspect ratio” is the length of a first axis of ananostructure divided by the average of the lengths of the second andthird axes of the nanostructure, where the second and third axes are thetwo axes whose lengths are most nearly equal to each other. For example,the aspect ratio for a perfect rod would be the length of its long axisdivided by the diameter of a cross-section perpendicular to (normal to)the long axis.

The term “heterostructure” when used with reference to nanostructuresrefers to nanostructures characterized by at least two different and/ordistinguishable material types. Typically, one region of thenanostructure comprises a first material type, while a second region ofthe nanostructure comprises a second material type. In anotherembodiment, the nanostructure comprises a core of a first material andat least one shell of a second (or third etc.) material, where thedifferent material types are distributed radially about the long axis ofa nanowire, a long axis of an arm of a branched nanocrystal, or thecenter of a nanocrystal, for example. A shell need not completely coverthe adjacent materials to be considered a shell or for the nanostructureto be considered a heterostructure. For example, a nanocrystalcharacterized by a core of one material covered with small islands of asecond material is a heterostructure. In other embodiments, thedifferent material types are distributed at different locations withinthe nanostructure. For example, material types can be distributed alongthe major (long) axis of a nanowire or along a long axis or arm of abranched nanocrystal. Different regions within a heterostructure cancomprise entirely different materials, or the different regions cancomprise a base material.

As used herein, a “nanostructure” is a structure having at least oneregion or characteristic dimension with a dimension of less than about500 nm, e.g., less than about 200 nm, less than about 100 nm, less thanabout 50 nm, or even less than about 20 nm. Typically, the region orcharacteristic dimension will be along the smallest axis of thestructure. Examples of such structures include nanowires, nanorods,nanotubes, branched nanocrystals, nanotetrapods, tripods, bipods,nanocrystals, nanodots, quantum dots, nanoparticles, branched tetrapods(e.g., inorganic dendrimers), and the like. Nanostructures can besubstantially homogeneous in material properties, or in otherembodiments can be heterogeneous (e.g., heterostructures).Nanostructures can be, for example, substantially crystalline,substantially monocrystalline, polycrystalline, amorphous, orcombinations thereof. In one aspect, one of the three dimensions of thenanostructure has a dimension of less than about 500 nm, for example,less than about 200 nm, less than about 100 nm, less than about 50 nm,or even less than about 20 mm.

As used herein, the term “nanowire” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) that includes at least one cross sectional dimension that isless than 500 nm, and preferably, less than 100 nm, and has an aspectratio (length:width) of greater than 10, preferably greater than 50, andmore preferably, greater than 100.

The nanowires of this invention can be substantially homogeneous inmaterial properties, or in other embodiments can be heterogeneous (e.g.nanowire heterostructures). The nanowires can be fabricated fromessentially any convenient material or materials, and can be, e.g.,substantially crystalline, substantially monocrystalline,polycrystalline, amorphous, or combinations thereof. Nanowires can havea variable diameter or can have a substantially uniform diameter, thatis, a diameter that shows a variance less than about 20% (e.g., lessthan about 10%, less than about 5%, or less than about 1%) over theregion of greatest variability and over a linear dimension of at least 5nm (e.g., at least 10 nm, at least 20 nm, or at least 50 nm). Typicallythe diameter is evaluated away from the ends of the nanowire (e.g., overthe central 20%, 40%, 50%, or 80% of the nanowire). A nanowire can bestraight or can be e.g., curved or bent, over the entire length of itslong axis or a portion thereof. In other embodiments, a nanowire or aportion thereof can exhibit two- or three-dimensional quantumconfinement.

Examples of such nanowires include semiconductor nanowires as describedin Published International Patent Application Nos. WO 02/17362, WO02/48701, and WO 01/03208, carbon nanotubes, and other elongatedconductive or semiconductive structures of like dimensions, which areincorporated herein by reference.

As used herein, the term “nanorod” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) similar to a nanowire, but having an aspect ratio (length:width)less than that of a nanowire. Note that two or more nanorods can becoupled together along their longitudinal axis so that the couplednanorods span all the way between electrodes. Alternatively, two or morenanorods can be substantially aligned along their longitudinal axis, butnot coupled together, such that a small gap exists between the ends ofthe two or more nanorods. In this case, electrons can flow from onenanorod to another by hopping from one nanorod to another to traversethe small gap. The two or more nanorods can be substantially aligned,such that they form a path by which electrons can travel betweenelectrodes.

A wide range of types of materials for nanowires, nanorods, nanotubesand nanoribbons can be used, including semiconductor material selectedfrom, e.g., Si, Ge, Sn, Se, Te, B, C (including diamond), P, B—C,B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN, BP, BAs, AlN,AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, ZnO, ZnS, ZnSe,ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS,GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr, CuI,AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂, CuGeP₃,CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Ti, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄, Al₂O₃,(Al, Ga, In)₂ (S, Se, Te)₃, Al₂CO, and an appropriate combination of twoor more such semiconductors.

The nanowires can also be formed from other materials such as metalssuch as gold, nickel, palladium, iradium, cobalt, chromium, aluminum,titanium, tin and the like, metal alloys, polymers, conductive polymers,ceramics, and/or combinations thereof. Other now known or laterdeveloped conducting or semiconductor materials can be employed.

Nanowires of the present invention may also be comprised of organicpolymers, ceramics, inorganic semiconductors such as carbides andnitrides, and oxides (such as TiO₂ or ZnO), carbon nanotubes,biologically derived compounds, e.g., fibrillar proteins, etc. or thelike. For example, in certain embodiments, inorganic nanowires areemployed, such as semiconductor nanowires. Semiconductor nanowires canbe comprised of a number of Group IV, Group III-V or Group II-VIsemiconductors or their oxides. In one embodiment, the nanowires mayinclude metallic conducting, semiconducting, carbide, nitride, or oxidematerials such as RuO₂, SiC, GaN, TiO₂, SnO₂, WC_(x), MoC_(x), ZrC,WN_(x), MoN_(x) etc. As used throughout, the subscript “x,” when used inchemical formulae, refers to a whole, positive integer (e.g., 1, 2, 3,4, 5, 6, 7, 8, 9, 10, etc). It is suitable that the nanowires be madefrom a material that is resistant to degradation in a weak acid so thatthe nanowires are compatible with the reactants of a variety ofdifferent fuel cells. Nanowires according to this invention canexpressly exclude carbon nanotubes, and, in certain embodiments, exclude“whiskers” or “nanowhiskers”, particularly whiskers having a diametergreater than 100 nm, or greater than about 200 nm.

In other aspects, the semiconductor may comprise a dopant from a groupconsisting of: a p-type dopant from Group III of the periodic table; ann-type dopant from Group V of the periodic table; a p-type dopantselected from a group consisting of: B, Al and In; an n-type dopantselected from a group consisting of: P, As and Sb; a p-type dopant fromGroup II of the periodic table; a p-type dopant selected from a groupconsisting of: Mg, Zn, Cd and Hg; a p-type dopant from Group IV of theperiodic table; a p-type dopant selected from a group consisting of: Cand Si.; or an n-type dopant selected from a group consisting of: Si,Ge, Sn, S, Se and Te. Other now known or later developed dopantmaterials can be employed.

Additionally, the nanowires or nanoribbons can include carbon nanotubes,or nanotubes formed of conductive or semiconductive organic polymermaterials, (e.g., pentacene, and transition metal oxides).

It should be understood that the spatial descriptions (e.g., “above”,“below”, “up”, “down”, “top”, “bottom”, etc.) made herein are forpurposes of illustration only, and that devices of the present inventioncan be spatially arranged in any orientation or manner.

Nanomaterials have been produced in a wide variety of different ways.For example, solution based, surfactant mediated crystal growth has beendescribed for producing spherical inorganic nanomaterials, e.g., quantumdots, as well as elongated nanomaterials, e.g., nanorods andnanotetrapods. Other methods have also been employed to producenanomaterials, including vapor phase methods. For example, siliconnanocrystals have been reportedly produced by laser pyrolysis of silanegas.

Other methods employ substrate based synthesis methods including, e.g.,low temperature synthesis methods for producing, e.g., ZnO nanowires asdescribed by Greene et al. (“Low-temperature wafer scale production ofZnO nanowire arrays,” L. Greene, M. Law, J. Goldberger, F. Kim, J.Johnson, Y. Zhang, R. Saykally, P. Yang, Angew. Chem. Int. Ed. 42,3031-3034, 2003), and higher temperature VLS methods that employcatalytic gold particles, e.g., that are deposited either as a colloidor as a thin film that forms a particle upon heating. Such VLS methodsof producing nanowires are described in, for example, PublishedInternational Patent Application No. WO 02/017362, the full disclosureof which is incorporated herein by reference in its entirety for allpurposes.

Nanostructures can be fabricated and their size can be controlled by anyof a number of convenient methods that can be adapted to differentmaterials. For example, synthesis of nanocrystals of various compositionis described in, e.g., Peng et al. (2000) “Shape Control of CdSeNanocrystals” Nature 404, 59-61; Puntes et al. (2001) “Colloidalnanocrystal shape and size control: The case of cobalt” Science 291,2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos et al. (Oct. 23, 2001)entitled “Process for forming shaped group III-V semiconductornanocrystals, and product formed using process;” U.S. Pat. No. 6,225,198to Alivisatos et al. (May 1, 2001) entitled “Process for forming shapedgroup II-VI semiconductor nanocrystals, and product formed usingprocess;” U.S. Pat. No. 5,505,928 to Alivisatos et al. (Apr. 9, 1996)entitled “Preparation of III-V semiconductor nanocrystals;” U.S. Pat.No. 5,751,018 to Alivisatos et al. (May 12, 1998) entitled“Semiconductor nanocrystals covalently bound to solid inorganic surfacesusing self-assembled monolayers;” U.S. Pat. No. 6,048,616 to Gallagheret al. (Apr. 11, 2000) entitled “Encapsulated quantum sized dopedsemiconductor particles and method of manufacturing same;” and U.S. Pat.No. 5,990,479 to Weiss et al. (Nov. 23, 1999) entitled “Organoluminescent semiconductor nanocrystal probes for biological applicationsand process for making and using such probes.”

Growth of nanowires having various aspect ratios, including nanowireswith controlled diameters, is described in, e.g., Gudiksen et al. (2000)“Diameter-selective synthesis of semiconductor nanowires” J. Am. Chem.Soc. 122, 8801-8802; Cui et al. (2001) “Diameter-controlled synthesis ofsingle-crystal silicon nanowires” Appl. Phys. Lett. 78, 2214-2216;Gudiksen et al. (2001) “Synthetic control of the diameter and length ofsingle crystal semiconductor nanowires” J. Phys. Chem. B 105, 4062-4064;Morales et al. (1998) “A laser ablation method for the synthesis ofcrystalline semiconductor nanowires” Science 279, 208-211; Duan et al.(2000) “General synthesis of compound semiconductor nanowires” Adv.Mater. 12, 298-302; Cui et al. (2000) “Doping and electrical transportin silicon nanowires” J. Phys. Chem. B 104, 5213-5216; Peng et al.(2000) “Shape control of CdSe nanocrystals” Nature 404, 59-61; Puntes etal. (2001) “Colloidal nanocrystal shape and size control: The case ofcobalt” Science 291, 2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos etal. (Oct. 23, 2001) entitled “Process for forming shaped group III-Vsemiconductor nanocrystals, and product formed using process;” U.S. Pat.No. 6,225,198 to Alivisatos et al. (May 1, 2001) entitled “Process forforming shaped group II-VI semiconductor nanocrystals, and productformed using process”; U.S. Pat. No. 6,036,774 to Lieber et al. (Mar.14, 2000) entitled “Method of producing metal oxide nanorods”; U.S. Pat.No. 5,897,945 to Lieber et al. (Apr. 27, 1999) entitled “Metal oxidenanorods”; U.S. Pat. No. 5,997,832 to Lieber et al. (Dec. 7, 1999)“Preparation of carbide nanorods;” Urbau et al. (2002) “Synthesis ofsingle-crystalline perovskite nanowires composed of barium titanate andstrontium titanate” J. Am. Chem. Soc., 124, 1186; and Yun et al. (2002)“Ferroelectric Properties of Individual Barium Titanate NanowiresInvestigated by Scanned Probe Microscopy” Nanoletters 2, 447.

In certain embodiments, the nanowires of the present invention areproduced by growing or synthesizing these elongated structures onsubstrate surfaces. By way of example, published U.S. Patent ApplicationNo. US-2003-0089899-A1 discloses methods of growing uniform populationsof semiconductor nanowires from gold colloids adhered to a solidsubstrate using vapor phase epitaxy. Greene et al. (“Low-temperaturewafer scale production of ZnO nanowire arrays”, L. Greene, M. Law, J.Goldberger, F. Kim, J. Johnson, Y. Zhang, R. Saykally, P. Yang, Angew.Chem. Int. Ed. 42, 3031-3034, 2003) discloses an alternate method ofsynthesizing nanowires using a solution based, lower temperature wiregrowth process. A variety of other methods are used to synthesize otherelongated nanomaterials, including the surfactant based syntheticmethods disclosed in U.S. Pat. Nos. 5,505,928, 6,225,198 and 6,306,736,for producing shorter nanomaterials, and the known methods for producingcarbon nanotubes, see, e.g., US-2002/0179434 to Dai et al., as well asmethods for growth of nanowires without the use of a growth substrate,see, e.g., Morales and Lieber, Science, V.279, p. 208 (Jan. 9, 1998). Asnoted herein, any or all of these different materials may be employed inproducing the nanowires for use in the invention. For some applications,a wide variety of group III-V, II-VI and group IV semiconductors may beutilized, depending upon the ultimate application of the substrate orarticle produced. In general, such semiconductor nanowires have beendescribed in, e.g., US-2003-0089899-A1, incorporated herein above.

Growth of branched nanowires (e.g., nanotetrapods, tripods, bipods, andbranched tetrapods) is described in, e.g., Jun et al. (2001) “Controlledsynthesis of multi-armed CdS nanorod architectures using monosurfactantsystem” J. Am. Chem. Soc. 123, 5150-5151; and Manna et al. (2000)“Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, andTetrapod-Shaped CdSe Nanocrystals” J. Am. Chem. Soc. 122, 12700-12706.

Synthesis of nanoparticles is described in, e.g., U.S. Pat. No.5,690,807 to Clark Jr. et al. (Nov. 25, 1997) entitled “Method forproducing semiconductor particles”; U.S. Pat. No. 6,136,156 to El-Shall,et al. (Oct. 24, 2000) entitled “Nanoparticles of silicon oxide alloys;”U.S. Pat. No. 6,413,489 to Ying et al. (Jul. 2, 2002) entitled“Synthesis of nanometer-sized particles by reverse micelle mediatedtechniques;” and Liu et al. (2001) “Sol-Gel Synthesis of Free-StandingFerroelectric Lead Zirconate Titanate Nanoparticles” J. Am. Chem. Soc.123, 4344. Synthesis of nanoparticles is also described in the abovecitations for growth of nanocrystals, nanowires, and branched nanowires,where the resulting nanostructures have an aspect ratio less than about1.5.

Synthesis of core-shell nanostructure heterostructures, namelynanocrystal and nanowire (e.g., nanorod) core-shell heterostructures,are described in, e.g., Peng et al. (1997) “Epitaxial growth of highlyluminescent CdSe/CdS core/shell nanocrystals with photostability andelectronic accessibility” J. Am. Chem. Soc. 119, 7019-7029; Dabbousi etal. (1997) “(CdSe)ZnS core-shell quantum dots: Synthesis andcharacterization of a size series of highly luminescent nanocrysallites”J. Phys. Chem. B 101, 9463-9475; Manna et al. (2002) “Epitaxial growthand photochemical annealing of graded CdS/ZnS shells on colloidal CdSenanorods” J. Am. Chem. Soc. 124, 7136-7145; and Cao et al. (2000)“Growth and properties of semiconductor core/shell nanocrystals withInAs cores” J. Am. Chem. Soc. 122, 9692-9702. Similar approaches can beapplied to growth of other core-shell nanostructures.

Growth of nanowire heterostructures in which the different materials aredistributed at different locations along the long axis of the nanowireis described in, e.g., Gudiksen et al. (2002) “Growth of nanowiresuperlattice structures for nanoscale photonics and electronics” Nature415, 617-620; Bjork et al. (2002) “One-dimensional steeplechase forelectrons realized” Nano Letters 2, 86-90; Wu et al. (2002)“Block-by-block growth of single-crystalline Si/SiGe superlatticenanowires” Nano Letters 2, 83-86; and U.S. Patent Application 60/370,095(Apr. 2, 2002) to Empedocles entitled “Nanowire heterostructures forencoding information.” Similar approaches can be applied to growth ofother heterostructures.

As described herein, and throughout co-assigned provisional PatentApplication No. 60/738,100, filed Nov. 21, 2005, the entire contents ofwhich are incorporated by reference herein, nanowire structures withmultiple shells can also be fabricated, such as, for example, aconducting inner core wire (which may or may not be doped) (e.g., toimpart the necessary conductivity for electron transport) and one ormore outer-shell layers that provide a suitable surface for bindingcatalyst (and/or polymer electrolyte). For example, in one embodiment, amulti-layer or multi-walled carbon nanotube (MWNT) can be formed inwhich the outermost shell layer is converted to silicon carbide toprovide a surface (SiC) to bind catalyst (and/or polymer electrolyte)and a conductive carbon nanotube core to impart the necessaryconductivity. In alternative embodiments, the core may consist ofheavily doped material such as doped silicon, and a shell of a carbide,nitride etc. material (e.g., SiC) may then be formed on the core. Theuse of silicon as the core material leverages the extensive experienceand infrastructure known for fabricating silicon nanowires. A carbideshell, such as SiC, WC, MoC or mixed carbide (e.g. WSiC) may be formedaround the core material using a controlled surface reaction. SiC, WCand MoC are known for their high conductivity and chemical stability. Inaddition, these materials have been shown to have catalytic propertiessimilar to those of precious metals, such as Pt, for methanol oxidation,and therefore may provide further performance enhancements in thenanowire bird's nest MEA. The precursor materials for the shell may bedeposited on the core nanowire surface (e.g., silicon) by atomic layerdeposition (ALD) and then converted to the carbide by high-temperaturecarbothermal reduction, for example.

FIG. 1A. shows a transmission electron micrograph (TEM) of a nanowirestructure 100 according to an embodiment of the present invention.Nanowire structure 100 comprises core 102, carbon-based layer 104 and,in suitable embodiments, comprises carbon-based structure 106 formed onthe carbon-based layer extending away from the core. The terms “core”and “nanowire core” are used interchangeably herein. Suitably, core 102comprises a semiconductor material, including but not limited to, thosesemiconductors disclosed throughout. For example, core 102 can compriseSi, B, SiC or GaN. Suitably the semiconductor core is highly doped. Inother embodiments, core 102 can comprise an inorganic oxide, aninorganic carbide or an inorganic nitride. Suitable inorganic oxidesinclude, but are not limited to, SiO₂, Al₂O₃, TiO₂, SnO₂, ZrO₂, HfO₂ andTa₂O₅. Suitable inorganic carbides include, but are not limited to, TiC,ZrC, HfC, NbC, WC, W₂C, MoC and Mo₂C. Suitable inorganic nitridesinclude, but are not limited to, TiN, ZrN, HfN, WN, MoN and BN. Core 102can also comprise a carbon nanofiber, e.g., a carbon nanotube or acarbon nanofibril.

In other embodiments, core 102 can comprise carbon, consist essentiallyof carbon or consist of carbon (i.e. consist only of carbon). Inembodiments where core 102 consists only of carbon, nanowire structure100 represents a purely carbon-based nanowire, where core 102 is carbonand carbon-based layer 104 is carbon in either amorphous and/orcrystalline forms (e.g., basal plane carbon in the form of graphenelayers or sheets). Such embodiments are distinct from carbon nanotubes,however, as core 102 is not hollow, but rather carbon-based. Inembodiments where core 102 comprises carbon or consists essentially ofcarbon, core 102 can also further comprise additional materials, such assemiconductors, metals, or other materials, as described herein. Inanother embodiment, core 102 can comprise a carbide, i.e. a mixture ofcarbon and an additional material as described herein. For example, core102 can comprise inorganic carbides, such as SiC, TiC, ZrC, HfC, NbC,WC, W₂C, MoC and Mo₂C. In other embodiments, core 102 is substantiallydevoid of carbon, that is will contain less than about 0.5% carbon,e.g., less than about 0.25%, less than about 0.1%, or suitably, nocarbon at all.

Core 102 of nanowire structure 100 can be prepared using any suitablemethod known in the art. For example, core 102 can be prepared frommetallic nucleation particles using chemical vapor deposition andrelated methods, such as those disclosed in Pan et al., “Systems andMethods for Nanowire Growth and Harvesting,” U.S. patent applicationSer. No. 11/117,703, filed Apr. 29, 2005, and Romano et al., “Methodsfor Nanowire Growth,” U.S. patent application Ser. No. 11/117,702, filedApr. 29, 2005, each of which is incorporated herein by reference, or asotherwise disclosed in other patents, patent applications and referencesdescribed herein. In general, core 102 will be grown on a suitablesubstrate material, such as Si or other semiconductor material. Insuitable embodiments, cores 102 are prepared from metallic nucleationparticles that are about 10 nm to about 100 nm in diameter. For example,cores 102 can be prepared from nucleation particles that are about 20 nmin diameter that have been deposited on a substrate surface at aparticle density of about 80 particles/μm² to about 400 particles/μm²,or from nucleation particles that are about 10 nm in diameter that havebeen deposited on a substrate at a particle density of about 4particles/μm² to about 40 particles/μm². When nucleation particles ofabout 20 nm in diameter are used (e.g., 20 nm gold nanoparticles),substantially pure nanowires with an average diameter of about 27 nm areproduced. Use of nucleation particles of about 10 nm in diameter resultin nanowires with an average diameter of about 15 nm. Suitableconditions for growing nanowires from 10 nm nucleation particles arereadily determined by those of skill in the art. Exemplary conditionsinclude SiH₄ flow at 20 to 40 standard cubic centimeters per minute(sccm) (e.g., about 40 sccm), H₂/He flow at 50/350, 200/200 and 400/0sccm (e.g., about 50/350) and total pressure of about 15, 30 and 45 Torr(e.g., about 15 Torr).

The formation of carbon-based layer 104 and carbon-based structure 106,when present, can occur while core 102 is still attached to thesubstrate material, or core 102 can be removed from the substratematerial for later processing, as described herein. Suitably, core 102will have a length of greater than about 50 nm to less than about 1 μm.In suitable such embodiments, core 102 will have a length of a few 100nms. In addition, core 102 will be at least about 1 nm to less thanabout 1 μm in cross-sectional diameter. Suitably, core 102 will have adiameter of about a few nms to 100's of nms, but generally less thanabout 500 nm.

As used herein, the term “carbon-based layer” 104 refers to a layer,coating, or other similar structure of carbon-comprising material eitherpartly or completely surrounding core 102. Carbon based layer 104 can bein the form of islands or sections of material on core 102, or cancompletely cover and surround core 102 in a substantially uniform layer.Carbon-based layer 104 can comprise carbon in any form, for example,substantially crystalline, substantially monocrystalline,polycrystalline, amorphous (i.e., non-crystalline), or combinationsthereof. In an embodiment, carbon-based layer 104 will be substantiallynon-crystalline or amorphous. As used herein, the terms non-crystallineand amorphous refer to carbon that lacks a distinct crystallinestructure and instead has a random arrangement of carbon atoms. In otherembodiments, amorphous, non-crystalline carbon-based layer 104 will besubstantially devoid of basal plane carbon. That is, carbon-based layer104 will contain less than about 0.5% basal plane carbon, e.g., lessthan about 0.25%, less than about 0.1%, or suitably, no basal planecarbon at all. Basal plane carbon refers to carbon in its characteristicbonded, crystalline structure found in graphene sheets and/or graphitelayers.

In an embodiment, carbon-based layer 104 is an interfacial carbidelayer. The term “interfacial carbide layer” refers to carbide that hasformed at the interface of core 102, where the surface is exposed to thesurrounding environment. Interfacial carbide layers can comprise anysuitable carbide, such as, but not limited to, SiC, TiC, ZrC, HfC, NbC,WC, Mo₂C and mixtures thereof. Interfacial carbide layers on the surfaceof core 102 can be of any thickness. In some embodiments, core 102 andcarbon-based layer 104 are both composed entirely of carbide. In otherembodiments, carbon-based layer 104 is greater than about 1 nm inthickness, e.g., about 1 nm to about 500 nm in thickness (i.e., theentire thickness of nanowire structure 100 can be a carbide). In otherembodiments, carbon-based layer 104 is on the order of a few angstroms(Å) to 10's of Å thick, surrounding core 102 which makes up theremaining thickness of the nanowire structure 100.

Carbon-based layer 104 is formed on core 102 by contacting core 102 withone or more carbon-comprising gases or gas mixtures at an elevatedtemperature. As the carbon-comprising gas contacts core 102, carbonprecipitates out of the gas phase and forms carbon-based layer 104 atthe interface of core 102 and the surrounding environment. In anembodiment, where core 102 is a semiconductor or like material,carbon-based layer 104 is in the form of a carbide layer formed on core102. The carbon-comprising gas can be in the form of a gas mixturecomprising several component cases. In another embodiment, in additionto a carbon-based gas, this gas mixture can also comprise one or morenoble gases, or other gases such as hydrogen, to maintain the partialpressure of the gas mixture and to control the carbide and nanographiteformation. These additional gases help to limit the amount of carboncomponent that precipitates out prior to contact with core 102. Suchgases, however, do not, and should not, contaminate or otherwiseinterfere with the formation of carbon-based layer 104. Suitablecarbon-based gases for use in the mixtures include, but are not limitedto, carbon monoxide, methane, ethane, propane, butane, ethylene,propylene and various derivatives and mixtures thereof. Contacting core102 with the carbon-comprising gas mixture can occur at any suitabletemperature determined by the ordinarily skilled artisan. In general,the temperature will be in the range of about 400° C. to about 1500° C.,or more suitably about 600° C. to about 1300° C. In other embodiments,as carbon-based layer 104 is formed, carbon begins to migrate toward thecenter of nanowire structure 100 and permeate the entire structure,including core 102. In such embodiments, the entire nanowire structure100 can therefore be substantially carbon-based throughout, e.g.,carbide, and in further embodiments, can be entirely carbon throughout.In an embodiment, the present invention provides a substantiallycarbon-based nanowire structure that is not a carbon nanotube, i.e. thatdoes not comprise basal plane carbon wrapped around a core (or hollowcenter), but rather, comprises amorphous, substantially non-crystallinecarbon layered on a core comprising, for example, a semiconductormaterial, or carbide.

As additional carbon precipitates on carbon-based layer 104 (suitably aninterfacial carbide layer), carbon-based structures 106 may begin toform on carbon-based layer 104 and chemically bond to the layer. Inother embodiments, carbon-based structures 106 are amorphous carbonfibers or nanowires extending from carbon-based layer 104. These carbonnanowires can comprise carbon, for example in the form of carbide, suchas SiC, or any suitably carbide described herein or known in the art. Inother embodiments, the nanowires can consist essentially of carbon, orin additional embodiments, can consist only of carbon. In suchembodiments the carbon-based structures will be substantially devoid ofany basal plane carbon.

In other embodiments, carbon-based structures 106 can be nanographiticplates formed on carbon-based layer 104 (suitably an interfacial carbidelayer) that chemically bond to the layer via the a-b lattice edges ofthe carbon crystals. In an embodiment, both nanowires and nanographiticplates can be formed at the same time as part of nanowire structure 100.The term a-b lattice edges refers to the dimensions in the plane of thecarbon-based structures 106 that are in the form of nanographitic plates(i.e., the flat dimensions of the plates). In an embodiment, thisformation and bonding can occur at the same temperature at whichcarbon-based layer 104 is formed. In other embodiments, carbon-basedlayer 104 can be formed at a temperature of about 400° C. to about 800°C., and then the temperature can be increased such that carbon-basedstructures 106 are formed, for example, at a temperature from about 800°C. to about 1300° C.

Though it is not required, generally, nanographitic plates are highlycrystallized, isolated graphene sheets or layers. The graphene layerswill generally not be interconnected (i.e., will not be connected viabonds normal to the plane of the layers as in graphite), and grow out ofthe plane of carbon-based layer 104 (suitably an interfacial carbidelayer), attached via the a-b edges of the graphenes to the carbon-basedlayer 104 and to each other. In other embodiments, however, graphenelayers can be interconnected as in the structure of graphite. Suitably,nanographitic plates will comprise less than about 100 graphene sheets,and more suitably, between about 2-15 graphenes. While the dimension ofnanographitic plates in the a-b plane (i.e., the plane of the graphenelayers) can be any size, generally they will be on the order of 10's to100's of nanometers. Suitably the nanographitic plates will be less thanabout 100 nm across in the a-b plane.

Carbon-based structures 106 (e.g., nanowires and/or nanographiticplates) generally extend away from core 102 and carbon-based layer 104 adistance of between about 1 nm and about 500 nm, suitably on the order afew nms to 10's of nms or even to a few 100 nms. The carbon-basedstructures 106 can be oriented at any angle, between 0° and 90°,relative to the major axis of core 102 (i.e., the long axis of thenanowire). Suitably, carbon-based structures 106 will be at an angle ofabout 45° to about 90° normal to the axis of core 102. FIG. 1B shows ahigh magnification TEM of carbon-based structures 106 (in the form ofnanographitic plates) extending away from carbon-based layer 104 andcore 102. The multiple layers of graphene sheets of the nanographiticplates can be seen.

FIG. 2 shows an X-ray diffraction pattern of a representative nanowirestructure 100 comprising core 102, carbon-based layer 104 (in the formof an interfacial carbide layer) and carbon-based structures 106 (in theform of nanographitic plates). The nanowire structure used in this x-rayanalysis was prepared using a tungsten oxide (WO₃) coated siliconnanowire, contacted with methane-comprising gas (see Examples sectionfor an exemplary preparation). The presence of graphite (C), silicon(Si), silicon carbide (SiC) and tungsten carbide (WC) are detected inthe diffraction peaks, indicating the presence of a silicon core, asilicon carbide interfacial layer, and a graphite (graphene)nanographitic plate(s).

In another embodiment, as represented in FIGS. 3A and 3B, the presentinvention provides an interconnected nanowire network 300, comprising aplurality of nanowire structures 100, wherein carbon-based structures106, in the form of nanographitic plates, attached to the variousnanowire cores connect the nanowire structures 100. When a plurality ofcores 102 are contacted with a carbon-comprising gas under conditions(e.g., elevated temperature) that favor the formation of carbon-basedlayers 104 (e.g., interfacial carbide layers) and carbon-basedstructures 106 as nanographitic plates, interconnected nanowire network300 forms. As carbon-based structures 106 are generated in the form ofnanographitic plates, graphenes extend out in the a-b direction awayfrom adjacent cores 102, until they overlap, or in many cases,interconnect and/or bond to adjacent carbon-based layers 104 (e.g.,interfacial carbide layers) and/or other carbon-based structures 106 inthe firm of nanographitic plates. The resulting interconnected nanowirenetwork 300, as shown in a scanning electron micrograph (SEM) (FIG. 3A)and high magnification SEM (FIG. 3B), has a densely-packed arrangementof nanowires and interconnecting nanographitic plates. This structure ofdensely packed nanowires, with or without interconnecting nanographiticplates, is also referred to throughout as a “bird's nest” structure.This arrangement takes the form of a porous structure, wherein the sizeof pores between the nanowires and nanographitic plates are suitablymesopores. As used herein the term “mesopores” refers to pores that arelarger than micropores (micropores are defined as less than about 2 nmin diameter), but smaller than macropores (macropores are defined asgreater than about 50 nm in diameter), and therefore have a pore size inthe range of greater than about 2 nm to less than about 50 nm indiameter. Suitably, interconnected nanowire network 300 will besubstantially free of micropores, that is, less than about 0.1% of thepores will be micropores (i.e., less than about 2 nm in diameter).

The mesoporous material formed by interconnected nanowire network 300can take the form of a membrane, particle or other similar porousstructure. In the form of a membrane, the mesoporous material willgenerally be a layer of interconnected nanowire network 300 comprising asuitable volume of the network, cut and shaped to the appropriate size.The membrane can be prepared at any overall thickness, depending on theultimate application, by varying the amount of starting material andreaction temperatures and times, as can be readily determined by thoseskilled in the art. Generally, the thickness of the mesoporous materialwill be on the order of 10's to 100's of nanometers, up to severalmicrons and even millimeters depending upon the final desiredapplication.

The creation of particles of the mesoporous material can be generated bygrinding, cutting, or otherwise breaking the interconnected nanowirestructure 300 into small pieces, each comprising the mesoporous materialnetwork. Techniques and devices for grinding, cutting or otherwisebreaking the interconnected nanowire structure 300 are well known by,and readily available to, those of skill in the art (e.g., mortar andpestle, rotary grinders, tumblers, and the like). These particles canthen be appropriately filtered or selected such that particles of only acertain size are used, depending upon the final application.

In another embodiment, as represented in flowchart 400 of FIG. 4, withreference to FIG. 1A, the present invention also provides methods ofmanufacturing a nanowire structure 100. In step 402 of FIG. 4, ananowire core 102 is heated. In step 404 of FIG. 4, the nanowire core102 is contacted with one or more carbon-comprising gases to form acarbon-based layer 104 (e.g., an interfacial carbide layer) on thenanowire core 102. The methods of manufacturing of the present inventioncan also further comprise step 406 of FIG. 4, in which at least onecarbon-based structure 106 (e.g., a nanowire and/or a nanographiticplate) is formed on carbon-based layer 104.

Heating step 402 will generally comprise heating core 102 to atemperature of greater than about 400° C., more suitably greater thanabout 600° C., and most suitably about 600° C. to about 1300° C. Asdiscussed throughout, contacting step 404 will comprise contacting core102 with a carbon-comprising gas, such as carbon monoxide, methane,ethane, propane, butane, ethylene or propylene. As shown in step 410 ofFIG. 4, nanowire core 102 can also be contacted with a noble or similargas (e.g., He, Ne, Ar, Kr, Xe, H₂ and the like) that helps to maintainthe partial pressure of the gas mixture to prevent the carbon fromprecipitating out of the gas phase too early in the process.

Flowchart 400 can also comprise optional step 408 of heating nanowirecore 102 to a second temperature in addition to the first temperatureutilized during heating step 402. In optional heating step 408, nanowirecore 102 and carbon-based layer 104 forming on core 102 can be heated toa second, higher temperature that allows, or in some cases enhances,formation of carbon-based structures 106 (e.g., nanographitic platesand/or nanowires). This second heating step 408 will suitably increasethe temperature of core 102 and carbon-based layer 104 to greater thanabout 800° C., more suitably to greater than about 1000° C., forexample, to about 1200-1300° C.

Flowchart 400 of FIG. 4 can also further comprise step 412, forming aprecursor coating on the nanowire core 102 prior to heating step 402.Examples of precursor coatings that can be generated on the nanowirecores 102 include oxides, including but are not limited to, TiO₂, ZrO₂,HfO₂, Nb₂O₃, Ta₂O₅, MoO₃ and WO₃.

In another embodiment of the present invention, as shown in flowchart500 of FIG. 5, with reference to FIGS. 1 and 3A, the present inventionalso provides methods of manufacturing an interconnected nanowirenetwork 300 (or bird's nest structure). In step 502 of FIG. 5, aplurality of nanowire cores 102 are dispersed in a liquid. Any suitableliquid that does not interact with the cores can be used, for examplewater or various alcohols. In step 504, nanowire cores 102 are filtered,for example, by placing the dispersed nanowire cores through afiltration funnel or similar device to remove the liquid from thenanowire cores, thereby forming a nanowire mat (not shown). The nanowiremat can then be dried. Dispersing 502 and filtration 504 steps help torandomize the nanowire cores 102 and aid in the production of theinterconnected network 300 by creating a disperse, random arrangement ofnanowire cores 102 prior to generation of the carbon-based structures106, suitably nanographitic plates. This dispersion and filtration areespecially helpful when preparing a membrane structure comprisinginterconnected nanowire network 300. As used herein, the term “nanowiremat” refers to a plurality of nanowires (i.e., more than one) that havebeen filtered so as to randomize the nanowires. A nanowire wire mat, forexample, will comprise a plurality of randomly oriented nanowires, lyingor disposed in a substantially planar structure that can then be used toform interconnected nanowire network 300. In step 506 of FIG. 5, thenanowire mat is then heated. In step 508 of FIG. 5, the nanowire mat iscontacted with carbon-comprising gas to form carbon-based layers 104,suitably interfacial carbide layers, on the cores 102. In step 510 ofFIG. 4, carbon-based structures 106, suitably nanographitic plates, areformed on the interfacial carbide layer, such that the platesinterconnect the nanowire cores 102 to generate the interconnectednanowire network 300.

Heating step 506 generally comprises heating the nanowire mat to atemperature of greater than about 400° C., more suitably greater thanabout 600° C., and most suitably about 600° C. to about 1300° C. Asdiscussed throughout, contacting step 508 comprises contacting thenanowire mat with a carbon-comprising gas, such as carbon monoxide,methane, ethane, propane, butane, ethylene or propylene. As shown instep 514 of FIG. 5, nanowire mat can also be contacted with a noble orsimilar gas (e.g., H, He, Ne, Ar, Kr, Xe, H₂ and the like).

Flowchart 500 can also comprise optional step 512 of heating thenanowire mat to a second temperature in addition to the firsttemperature utilized during heating step 506. In optional heating step512, nanowire mat and carbon-based layer 104 forming on cores 102 can beheated to a second, higher temperature that allows, or in some casesenhances, formation of nanographitic plates (or nanowires). This secondheating step 512 suitably increases the temperature of the mat, coresand interfacial carbide layers to greater than about 800° C., moresuitably to greater than about 1000° C., for example, to about1200-1300° C.

Flowchart 500 of FIG. 5 can also further comprise step 516, forming aprecursor coating on the nanowire cores 102 prior to heating step 506,and suitably, prior to dispersing step 502. Examples of precursorcoatings that can be generated on the nanowire cores include oxides,including but not limited to, TiO₂, ZrO₂, HfO₂, Nb₂O₃, Ta₂O₅, MoO₃ andWO₃.

Applications of Nanowire Structures and Interconnected Nanowire Networks

Having described the formation of free-standing nanowire structures 100,and interconnected nanowire networks 300 in detail above, it will bereadily apparent to one of ordinary skill in the art that such nanowiresand networks can also be formed on the surface of a support. Forexample, the nanowires and networks can be formed on the surface of aconventional fiber network, a flat or irregular surface, or a foamstructure.

Fuel Cell Applications

The nanowire structures and interconnected nanowire networks of thepresent invention can also be used in various fuel cell applications andconfigurations. For example catalysts comprising nanowires orinterconnected nanowire networks and active catalytic nanoparticlesdispersed on the surface of the nanowires/networks can be generated.Exemplary catalytic nanoparticles include, but are not limited to, Pt,Pd, Ru, Rh, Re, No, Fe, Co, Ag, Au, Cu, Zn and Sn, as well as metalalloy nanoparticles comprising two or more of such elements. Thesecatalysts can be used as fuel cell cathodes, for example, a cathodecomprising a nanowire or interconnected nanowire network and Ptcatalytic nanoparticles with a diameter from about 2 nm to about 10 nm,or more suitably from about 3 nm to about 5 nm can be created. Thecatalysts can also be used as fuel cell anodes, for example, by usingcatalytic Pt—Ru nanoparticles on the order of about 2 nm to about 10 nm,or more suitably, from about 3 nm to about 5 nm in diameter. Inexemplary anode catalysts, the Pt—Ru nanoparticles will have an atomicratio of Pt:Ru of about 0.1 to about 20, or more suitably about 1 toabout 3.

The present invention also provides membrane electrode assemblies (MEA)comprising the cathode catalysts and anode catalysts described herein,and also a membrane (e.g., a NAFION® membrane, DuPont, Wilmington,Del.). Such MEAs can be constructed using well known methods in the art,for example as set forth in U.S. Pat. Nos. 6,933,033; 6,926,985; and6,875,537, the disclosures of each of which are incorporated herein byreference in their entireties. In exemplary embodiments, the membranewill be disposed on one side with a cathode catalyst and on the otherside an anode catalyst. Fuel cells comprising such MEAs, as well as gasdiffusion layers (e.g., carbon fiber cloth), bipolar plates and endplates (e.g., machined graphite or molded conducting polymer composites)can also be constructed, as is well known in the art. Exemplary fuelcells that can be constructed using the nanowires and interconnectednanowire networks disclosed herein include proexchange membrane fuelcells (PEMF) and direct methanol fuel cells (DMFC). The nanowires andinterconnected nanowire networks can also be used to generate anodes andcathodes, for example for use in lithium batteries and electrochemicalcapacitors. The components and construction of such batteries andcapacitors is well known in the art.

In one embodiment of the invention, the nanowire portion of the anode(and/or cathode) electrode of the invention may be synthesized on agrowth substrate, and then transferred and incorporated into themembrane electrode assembly structure of the fuel cell. For example, incertain aspects, inorganic semiconductor or semiconductor oxidenanowires are grown on the surface of a growth substrate using acolloidal catalyst based VLS synthesis method described herein and knownin the art. In accordance with this synthesis technique, the colloidalcatalyst (e.g., gold, platinum etc. particles) is deposited upon thedesired surface of the substrate. The substrate including the colloidalcatalyst is then subjected to the synthesis process which generatesnanowires attached to the surface of the substrate. Other syntheticmethods include the use of thin catalyst films, e.g., 50 nm or less,deposited over the surface of the substrate. The heat of the VLS processthen melts the film to form small droplets of catalyst that forms thenanowires. Typically, this latter method may be employed where fiberdiameter homogeneity is less critical to the ultimate application.Typically, growth catalysts comprise metals, e.g., gold or platinum, andmay be electroplated or evaporated onto the surface of the substrate ordeposited in any of a number of other well known metal depositiontechniques, e.g., sputtering etc. In the case of colloid deposition, thecolloids are typically deposited by first treating the surface of thesubstrate so that the colloids adhere to the surface. Such treatmentsinclude those that have been described in detail previously, i.e.,polylysine treatment, etc. The substrate with the treated surface isthen immersed in a suspension of colloid.

Following growth of the nanowires, the nanowires are then harvested fromtheir synthesis location. The free standing nanowires are thenintroduced into or deposited upon the relevant surface of the fuel cellcomponent such as the bipolar plate(s) or proton exchange membrane, forexample, by spray/brush painting, solution coating, casting,electrolytic deposition, filtering a fluid suspension of the nanowires,and combinations thereof. For example, such deposition may simplyinvolve immersing the component of interest (e.g., one or more of thebipolar plates or the proton exchange membrane) into a suspension ofsuch nanowires, or may additionally involve pre-treating all or portionsof the component to functionalize the surface or surface portions forwire attachment. As described herein, the nanowires may also beintroduced into a solution (e.g., methanol or water), filtered (e.g.,vacuum filtered over a polyvinylidene fluoride (PVDF) membrane) to givethem a dense, intertwined mat or “bird's nest structure,” removed fromthe filter after drying and washing, and then heat treated (e.g.,annealed) at high temperatures. The resulting porous sheet of nanowires(whether interconnected with nanographitic plates or note) can then beincorporated into the membrane electrode assembly of the fuel cell. Avariety of other deposition methods, e.g., as described in U.S. PatentApplication Publication No. 20050066883, published Mar. 31, 2005, andU.S. Pat. No. 6,962,823, the full disclosures of which are incorporatedherein by reference in their entirety for all purposes, can also beused. The nanowires may also be grown directly on one or more of thefuel cell components such as one or more of the bipolar plates and/orproton exchange membrane.

Typically, as shown in FIG. 6, a fuel cell 600 generally comprises ananode electrode 602, a cathode electrode 604, and a proton exchangemembrane (PEM) 606. The assembly of these three components is generallyreferred to as a membrane electrode assembly (MEA). As describedpreviously, if methanol is used as fuel, liquid methanol (CH₃OH) isoxidized in the presence of water at the anode 602 generating CO₂,hydrogen ions and the electrons that travel through the external circuit608 as the electric output of the fuel cell. The hydrogen ions travelthrough the electrolyte membrane 606 and react with oxygen from the airand the electrons from the external circuit 608 to form water at thecathode completing the circuit. Anode and cathode electrodes 602, 604each contact bipolar plates 610, 612, respectively. The bipolar plates610, 612 typically have channels and/or grooves in their surfaces thatdistribute fuel and oxidant to their respective catalyst electrodes,allow the waste, e.g., water and CO₂ to get out, and may also containconduits for heat transfer. Typically, bipolar plates are highlyelectrically conductive and can be made from graphite, metals,conductive polymers, and alloys and composites thereof. Materials suchas stainless steel, aluminum alloys, carbon and composites, with orwithout coatings, are good viable options for bipolar end plates in PEMfuel cells. Bipolar plates can also be formed from composite materialscomprising highly-conductive or semiconducting nanowires incorporated inthe composite structure (e.g., metal, conductive polymer etc.). Theshape and size of the components of the fuel cell can vary over a widerange depending on the particular design.

In one embodiment of the present invention, nanowires may be deposited(e.g., grown) on one or more of the bipolar plates to provide a highsurface area electrode plate with low flow resistance for methanol (orother fuel cell gas or liquid reactants) and waste products through it.A more complete description of nanowire structures having enhancedsurface areas, as well as to the use of such nanowires and nanowirestructures in various high surface area applications, is provided inU.S. patent application Ser. No. 10/792,402 entitled “Nanofiber Surfacesfor use in Enhanced Surface Area Applications,” filed Mar. 2, 2004, theentire contents of which are incorporated by reference herein.

At present, the most commonly used electrode catalyst is Pt or Pt:Ruparticles 702 supported on carbon particles 704 (e.g., made from carbonblack) which are dispersed in an electrolyte film 706 as shown in theexpanded view of the anode 602 in FIG. 7A. One of the challenges in thecommercialization of proton exchange membrane fuel cells (PEMFCs) is thehigh cost of the precious metals used as the catalyst (e.g., Pt or Ru).Decreasing the amount of Pt used in a PEMFC by increasing theutilization efficiency of Pt has been one of the major concerns duringthe past decade. To effectively utilize the Pt catalyst, the Pt shouldhave simultaneous contact to the reactant gases (or reactant solutionsor liquids), the electrolyte (e.g., proton conducting film), and thecarbon particles (e.g., electron-conducting element). As shown in FIG.7B, an effective electrode in a fuel cell requires a 4-phase-contact 708in the catalyst layer between the reactant gases/liquid, active metalparticles, carbon support 702, 704, and the electrolyte 706. A preferredcatalyst layer allows the facile transport of reactant gases (e.g.,methanol, MeOH:H₂O, hydrogen and/or oxygen), solutions, or liquids,facile transport of electrons to/from the external circuit and protonsto/from the proton exchange membrane.

The carbon particles conduct electrons and the perfluorosulfonateionomer (e.g., NAFION®) conducts protons. As noted previously, inconventional packed particle composite systems as shown in FIGS. 7A-B,there is a significant portion of Pt (or Pt:Ru) that is isolated fromthe external circuit and/or the PEM, resulting in a low Pt utilization.For example, current packed particle composites only utilize about 20 to30% of the catalyst particles. The inaccessibility to some catalystsites can be due, for example, to the fact that the necessary additionof the solubilized perfluorosulfonate ionomer (e.g., NAFION®) for protontransport tends to wash away or isolate carbon particles in the catalystlayer, leading to poor electron transport. Thus, most DMFCs whichutilize packed particle composite structures are highly inefficient.

Due to their unique structural, mechanical, and electrical properties,the inventors of the present application have discovered that nanowirescan be used to replace traditional carbon particles in PEMFCs as thecatalyst support and electron conducting medium to make MEAs. Becausethe generation of surface functional groups on nanowires, e.g.,nanowires such as SiC or GaN, is relatively straightforward, catalystnanoparticles such as Pt and/or Pt:Ru (as well as a proton conductingpolymer (e.g., NAFION®)), can be facilely deposited on the nanowires,e.g., without agglomeration of the particles. Each catalyst particle isthen directly connected to the anode (and cathode) through the nanowirecore. The multiple electrical connectivity of the interconnectednanowires secures the electronic route from Pt to the electronconducting layer. The use of nanowires and the resulting guaranteedelectronic pathway eliminate the previously mentioned problem withconventional PEMFC strategies where the proton conducting medium (e.g.,NAFION®) would isolate the carbon particles in the electrode layer.Eliminating the isolation of the carbon particles supporting theelectrode layer improves the utilization rate of Pt.

As shown now with reference to FIG. 8A, a nanowire-based fuel cell isshown which includes an anode bipolar electrode plate 802, a cathodebipolar electrode plate 804, a proton exchange membrane 806, an anodeelectrode 808, a cathode electrode 810, and an interconnecting networkof nanowires 812 positioned between both the anode electrode 808 andcathode electrode 810 on one side, and the proton exchange membrane 806on the other side of the fuel cell. Generally, a plurality of fuel cellsor MEAs as shown in FIG. 8A can be combined to form a fuel cell stack asshown, for example, in FIG. 8B having separate anode electrodes 808, 820and cathode electrodes 810, 822 separated by respective proton exchangemembranes 806 and 806′, respectively. The cells within the stacks areconnected in series by virtue of the bipolar plates 802, 804, 818, and824 such that the voltages of the individual fuel cells are additive.

As shown in FIGS. 8A, 9A and in the SEM image of FIG. 10, the nanowires816 in the nanowire networks 812 each are physically and/or electricallyconnected to one or more other wires in the network to form an open,highly branched, porous, intertwined structure, with low overalldiffusion resistance for reactants and waste diffusion, high structuralstability and high electrical connectivity for the electrons to ensurehigh catalytic efficiency, thus leading to high power density and loweroverall cost. It is important to note that even if two wires are not inactual direct physical contact with each other (or with a catalystparticle), it is possible that at some small distance apart, they maystill be able to transfer charges (e.g., be in electrical contact).Preferentially, each nanowire is physically and/or electricallyconnected to at least one or more other nanowires in the network. Themultiple connectivity of the nanowires ensures that if one wire breaksor is damaged in the system, for example, that all points along the wirestill connect to the anode (and cathode) electrode along different paths(e.g., via other nanowires in the network). This provides substantiallyimproved electrical connectivity and stability as compared to previouspacked particle composite structures. The wires may extend all the way(or only part way) between the anode (and cathode) bipolar plate and theproton exchange membrane. In the case where the wires do not extend allthe way between a bipolar plate and the membrane, the wires may extendfrom the bipolar plate toward the membrane, but not reach the membrane,and the polymer electrolyte can extend from the membrane toward thebipolar plate, but not reach the bipolar plate (but not the other wayaround) to ensure that electrons are efficiently transferred to theanode, and protons are transferred towards the cathode.

The nanowires in the nanowire network may optionally have a branchedstructure and include a plurality of nodules 1100 which extend from sidesurfaces of the nanowire as shown in FIG. 11 and in the SEM image ofFIG. 12. The nodules 1100 on the sides of the nanowire core can furtherincrease available surface area for catalysis without substantiallyimpacting the connectivity or porosity of the nanowire network.

The nanowires 816 are dispersed in a polymer electrolyte material 815(e.g., see FIG. 9A) which coats the surface of nanowires in the branchednanowire network to provide sufficient contact points for proton (e.g.,H+) transport. Polymer electrolytes can be made from a variety ofpolymers including, for example, polyethylene oxide, poly (ethylenesuccinate), poly (β-propiolactone), and sulfonated fluoropolymers suchas NAFION® (commercially available from DuPont Chemicals, Wilmington). Asuitable cation exchange membrane is described in U.S. Pat. No.5,399,184, for example, the disclosure of which is incorporated hereinby reference. Alternatively, the proton conductive membrane can be anexpanded membrane with a porous microstructure where an ion exchangematerial impregnates the membrane effectively filling the interiorvolume of the membrane. U.S. Pat. No. 5,635,041, incorporated herein byreference, describes such a membrane formed from expandedpolytetrafluoroethylene (PTFE). The expanded PTFE membrane has amicrostructure of nodes interconnected by fibrils. Similar structuresare described in U.S. Pat. No. 4,849,311, the disclosure of which isincorporated herein by reference.

The porous structure of the interconnected nanowire network provides anopen (non-tortuous) diffusion path for fuel cell reactants to thecatalyst (e.g., catalyst particles 814) deposited on the nanowires 816.The void spaces between the interconnected nanowires form a highlyporous structure. The effective pore size will generally depend upon thedensity of the nanowire population, as well as the thickness ofelectrolyte layer, and to some extent, the width of the nanowires used.All of these parameters are readily varied to yield a nanowire networkhaving a desired effective porosity. For example, preferred nanowirenetworks have a porosity adequate to provide for an even flow ofreactants while maintaining adequate electrical conductivity andmechanical strength. Also, the porosity of the nanowire network providesfor water management within the cell. The branched nanowire networkpreferably is sufficiently porous to pass fuel gases and water vaporthrough it without providing a site for water condensation that wouldblock the pores of the network and prevent vapor transport. The meanpore size generally ranges from about 0.002 microns to about 10.0microns, e.g., less than about 1 μm, e.g., less than about 0.2 μm, e.g.,less than about 0.02 μm, e.g., between about 0.002 μm and 0.02 μm, e.g.,between about 0.005 and 0.01 μm. The total porosity of the branchednanowire structure may be easily controlled between about 30% to 95%,for example, e.g., between about 40% to 60%, while still ensuringelectrical connectivity to the anode and cathode electrodes.

The nanowires 816 which form the interconnected nanowire networks 812may optionally be fused or cross-linked at the points where the variouswires contact each other, to create a more stable, robust andpotentially rigid membrane electrode assembly. The nanowires may alsoinclude surface chemical groups that may form chemical cross-links inorder to cross-link the underlying nanowires. For example, the nanowiresmay be cross-linked or fused together by depositing a small amount ofconducting or semiconducting material at their cross-points. Forexample, SiC nanowires (or, e.g., carbon nanotube nanowires having a SiCshell layer) can be cross-linked by depositing amorphous orpolycrystalline SiC at their cross-points. FIG. 13 is an SEM micrographshowing a plurality of silicon nanowires which have been fused togetherusing deposited polysilicon at their cross-points. One of skill in theart will appreciate that other metals, semimetals, semiconductors, andsemiconductor oxides could also be used to cross-link theseintersections.

In another aspect of the present invention, shown with reference to FIG.9B, nanowires 816′ may be provided as a parallel array of aligned wireshaving electrolyte 815′ interspersed between the free spaces between thealigned wires. In this particular implementation of the presentinvention, the parallel array of nanowires is preferably synthesized insitu, e.g., on the surface of the bipolar electrode plate(s) 802 and/or804 (and/or the proton exchange membrane 806). It is to be understoodthat the randomly oriented, interconnected network 812 of wires 816shown in FIGS. 8A, 9A and 10 and described above can also be grown insitu directly on the bipolar plates 802, 804 (and/or proton exchangemembrane) using the techniques described herein. For example, inorganicsemiconductor or semiconductor oxide nanowires may be grown directly onthe surface of the electrode plate using a colloidal catalyst based VLSsynthesis method described herein. In accordance with this synthesistechnique, the colloidal catalyst is deposited upon the desired surfaceof the bipolar plate. The bipolar plate including the colloidal catalystis then subjected to the synthesis process which generates nanowiresattached to the surface of the plate. Other synthetic methods includethe use of thin catalyst films, e.g., 50 nm or less, deposited over thesurface of the bipolar plate. The heat of the VLS process then melts thefilm to form small droplets of catalyst that forms the nanowires.Typically, this latter method may be employed where wire diameterhomogeneity is less critical to the ultimate application. Typically,catalysts comprise metals, e.g., gold or platinum, and may beelectroplated or evaporated onto the surface of the electrode plate ordeposited in any of a number of other well known metal depositiontechniques, e.g., sputtering etc. In the case of colloid deposition thecolloids are typically deposited by first treating the surface of theelectrode plate so that the colloids adhere to the surface. The platewith the treated surface is then immersed in a suspension of colloid.

In another aspect of the invention, the anode electrode 808 (and cathodeelectrode 810) may include a conductive grid or mesh made from any of avariety of solid or semisolid materials such as organic materials, e.g.,conductive polymers, carbon sheets, etc., inorganic materials, e.g.,semiconductors, metals such as gold, semimetals, as well as compositesof any or all of these, upon which the nanowires 816 may be attached,but through which apertures exist. Such meshes provide relativelyconsistent surfaces in a ready available commercial format with welldefined screen/pore and wire sizes. A wide variety of metal meshes arereadily commercially available in a variety of such screen/pore and wiresizes. Alternatively, metal substrates may be provided as perforatedplates, e.g., solid metal sheets through which apertures have beenfabricated. Fabricating apertures in metal plates may be accomplished byany of a number of means. For example relatively small apertures, e.g.,less than 100 μm in diameter, may be fabricated using lithographic andpreferably photolithographic techniques. Similarly, such apertures maybe fabricated using laser based techniques, e.g., ablation, laserdrilling, etc. For larger apertures, e.g., greater than 50-100 μm, moreconventional metal fabrication techniques may be employed, e.g.,stamping, drilling or the like. As formed, the metal grids or mesheswith the nanowires formed or deposited thereon by the methods disclosedherein may be deposited on the proton exchange membrane, bipolarplate(s), and or embedded within one or more of the electrode layers toprovide a porous network with a high surface area nanowire catalystsupport attached thereto for efficient catalysis. Other examples of avariety of grids or meshes with nanowires deposited thereon which can beused in the present invention are fully disclosed in U.S. patentapplication Ser. No. 10/941,746, entitled “Porous Substrates, Articles,Systems and Compositions Comprising Nanofibers and Methods of Their Useand Production,” filed on Sep. 15, 2004, the entire disclosure of whichis incorporated by reference herein.

The nanowire network thus formed by any of the methods described hereinis employed as the support for the subsequent metal (e.g., platinum,ruthenium, gold, or other metal) catalyst, which may be coated ordeposited, for example, on the nanowires. See e.g., FIG. 14. Appropriatecatalysts for fuel cells generally depend on the reactants selected. Forexample, the metallic catalyst (also called catalyst metals throughout)may be selected from the group comprising, but not limited to, one ormore of platinum (Pt), ruthenium (Ru), iron (Fe), cobalt (Co), gold(Au), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn),technetium (Tc), rhenium (Re), osmium (Os), rhodium (Rh), iridium (Ir),nickel (Ni), palladium (Pd), copper (Cu), silver (Ag), zinc (Zn), tin(Sn), aluminum (Al), and combinations and alloys thereof (such asbimetallic Pt:Ru nanoparticles). Suitable catalyst materials foroxidation of hydrogen or methanol fuels specifically include metals suchas, for example, Pd, Pt, Ru, Rh and alloys thereof.

FIG. 15A shows a transmission electron micrograph (TEM) of platinum (Pt)nanoparticles having an average diameter of about 1.61±0.31 nm andprepared in accordance with the present invention. FIG. 15B shows anX-ray diffraction pattern of these nanoparticles. The X-ray diffractionshows a characteristic peak at a value of 45° on the 2-theta scaleindicating that the nanoparticles are crystallized platinum metal.

FIG. 16A shows a TEM of platinum-ruthenium (Pt—Ru) alloy nanoparticleshaving an average diameter of about 1.66±0.33 nm and prepared inaccordance with the present invention. The X-ray diffraction in FIG. 16Bshows a peak at a value of 45° on the 2-theta scale indicating that thenanoparticles are crystallized Pt—Ru alloy.

The metal catalyst may be deposited or otherwise associated with thenanowire surface as a thin film (e.g., less than about 10 angstroms inthickness) (or a series of catalyst particles) using a variety ofcatalyst deposition techniques including, for example, chemical vapordeposition, electrochemical deposition (e.g., electroplating orelectroless chemical plating), physical vapor deposition, solutionimpregnation and precipitation, colloid particle absorption anddeposition, atomic layer deposition, and combinations thereof. Theamount of the catalyst metal coated by the methods described herein ispreferably in the range of about 0.5%-85% by weight, suitably about10%-85%, more suitably about 20-40% by weight, based on the total amountof catalyst metal and nanowire material.

Alternatively, in one embodiment, as shown with reference to FIGS. 8Aand 9A-B, the catalyst may be deposited on the nanowire surfaceplurality of nanometer-sized metallic catalyst particles 814 (e.g.,between about 1 and 50 nm in diameter, e.g., less than about 10 nm indiameter, e.g., between about 1 and 5 nm in diameter), in solution. Byderivatizing the nanowire external surface with one or more functionallinker moieties (e.g., a chemically reactive group) such as one or morecarboxylic acid groups, nitric acid groups, hydroxyl groups, aminegroups, sulfonic acid groups, and the like, the nanoparticles bind tothe surface of the nanowires. The catalysts particles (or film) can beattached to the wires either uniformly or non-uniformly. The catalystparticles can be spherical, semi-spherical or non-spherical. Thecatalyst particles can form islands on the surface of the nanowires orcan form a continuous coating on the surface of the nanowire such as ina core-shell arrangement, or stripes or rings along the length of thenanowire, etc. The catalyst particles may be attached to the nanowiresurface before or after the nanowire network is incorporated/depositedinto the MEA of the fuel cell. In one embodiment, the catalyst particlesmay be selected from a population of catalyst particles having a uniformsize distribution of less than about 50%, for example, less than about30%, for example, less than about 20%.

FIG. 17 shows two TEM images (110,000 times magnification (left image)and 67,044 times magnification (right image)) of Pt—Ru alloynanoparticles supported on the surface of carbon black-Cabot VULCAN®XC72, showing uniform distribution of the nanoparticles on the surfaceof carbon black without any observable agglomeration. Measurement ofamount deposited on the carbon black versus amount of alloy added to thesurface indicated a loading efficiency of about 24.3% of Pt—Ru alloy.

FIG. 18 shows an X-ray diffraction pattern recorded fromcarbon-supported Pt—Ru catalysts prepared in accordance with the presentinvention (top curve) as compared to a commercially available Pt—Rucatalysts (bottom curve). The lower relative intensity and higher2-theta position of the catalyst of the present invention (i.e., near45°, versus near 40° for commercial catalysts) were due to the smallerdiameter of the fabricated nanoparticles. FIG. 19 represents a TEM of Ptnanoparticles prepared by the methods of the present invention depositedon Si nanowires.

FIG. 20 shows TEM images of deposition of Pt—Ru nanoparticles on thesurface of nanographite coated Si nanowires (110,000 times magnification(left image) and 330,000 times magnification (right image)). As thenanowires are coated with carbon, the deposition methods used to depositnanoparticles on carbon black described above were used. The TEM imagesin FIG. 20 show uniform deposition of 1.67 nm Pt—Ru nanoparticlesdeposited on the nanographite coated nanowires at high density. Thenumber of nanoparticles deposited on the nanowires compared to thestarting number demonstrated a deposition efficiency of greater thanabout 20%.

When a chemical linker molecule is used to bind the catalyst to thenanowire, the chemical linker can be selected to promote electricalconnection between the catalyst and the wire, or the chemical linker canbe subsequently removed to promote electrical connection. For example,heat, vacuum, chemical agents or a combination thereof, can optionallybe applied to the nanowires to cause the linker molecule to be removedto place the catalyst in direct physical contact with the wire to form asolid electrical connection between the catalyst particles and thenanowire. The structure can also be heated to anneal the interfacebetween the catalyst and the wire in order to improve the electricalcontact therebetween. Appropriate temperatures and heating conditionsare well known to those of skill in the art.

The present invention also provides methods for preparing one or morenanowires comprising one or more catalyst metals associated with thenanowires. An exemplary embodiment of such methods is shown in flowchart2100 in FIG. 21. Suitably, as shown in FIG. 21, in step 2102, one ormore nanowires are dispersed in a solution. In step 2104, one or morecatalyst metals are then added to the solution, and in step 2106, thesolution is refluxed, whereby the catalyst metals become associated withthe nanowires. Suitably, the catalyst meals are added as a solutioncomprising one or more catalyst metal nanoparticles. As discussedthroughout, exemplary catalyst metals include, but are not limited to,chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), technetium(Tc), rhenium (Re), iron (Fe), ruthenium (Ru), osmium (Os), cobalt (Co),rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), platinum (Pt),copper (Cu), silver (Ag), gold (Au), zinc (Zn), tin (Sn), aluminum (Al),and combinations thereof. Suitably Pt or PtRu are used as catalystmetals.

Any suitable solution can be used for dispersion of the nanowires andthen subsequent refluxing. Exemplary solutions include organic solventssuch as ethylene glycol, as well as alcohols and aqueous-basedsolutions.

As used herein, “refluxing” means adding heat to a nanowire solution(heating) such that it boils, thereby driving volatile liquids from thesolution and leaving catalyst metal-associated nanowires behind in thecontainer in which the nanowire solution was heated. For example,refluxing can comprise heating the nanowire solution for about 10minutes to about 100 minutes, suitably about 20, 30, 40, 50, 60, 70, 80,or 90 minutes, at a temperature at which the solution boils. Typically,solutions of nanowires will boil at about 70° C. to about 200° C., forexample, about 90° C., 100° C., 110° C., 120° C., 130° C., 140° C., 150°C., 170° C., or about 190° C.

As discussed throughout, in exemplary embodiments, the nanowires arederivatized with at least a first functional group which binds thecatalyst metal, for example, a nitric acid, a carboxylic acid group, ahydroxyl group, an amine group, and a sulfonic acid group.

Following refluxing the nanowire solution, as shown in FIG. 21, in step2108, the catalyst metal-associated nanowires are filtered to generate asolid nanowire dispersion with associated catalyst metals, and thendried in step 2110.

Suitably the nanowires that are utilized in the refluxing methods of thepresent invention are the various nanowire structures and networksdescribed throughout.

In addition to the conductive catalyst particles, fillers can be used toalter the physical properties of the nanowire composite structuresuseful in the invention. Appropriate fillers include, e.g. silica(SiO₂), powdered polytetrafluoroethylene and graphite fluoride (CFn).The polymer films preferably can include up to about 20 percent byweight fillers, and more preferably from about 2 to about 10 percent byweight fillers. The fillers are generally in the form of particles.

Following catalyst deposition, a proton conducting polymer such asNAFION® may optionally be deposited on the nanowire surface betweencatalyst particle sites, for example, by functionalizing the surface ofthe nanowire with a second functional group (different from the catalystfunctional group, when used) that preferentially binds the electrolyteor which promotes consistent and/or controlled wetting. The polymer caneither be a continuous or discontinuous film on the surface of thenanowire. For example, the polymer electrolyte can be uniformly wettedon the surface of the wires, or can form point-contacts along the lengthof the wire. The nanowires may be functionalized with a sulfonatedhydrocarbon molecule, a fluorocarbon molecule, a short chain polymer ofboth types of molecules, or a branched hydrocarbon chain which may beattached to the nanowire surface via silane chemistry. Those of skill inthe art will be familiar with numerous functionalizations andfunctionalization techniques which are optionally used herein (e.g.,similar to those used in construction of separation columns, bio-assays,etc.). Alternatively, instead of binding ionomer to the nanowiresthrough a chemical binding moiety, the nanowires may be directlyfunctionalized to make them proton conductive. For example, thenanowires may be functionalized with a surface coating such as aperfluorinated sulfonated hydrocarbon using well-known functionalizationchemistries.

For example, details regarding relevant moiety and other chemistries, aswell as methods for construction/use of such, can be found, e.g., inHermanson Bioconjugate Techniques Academic Press (1996), Kirk-OthmerConcise Encyclopedia of Chemical Technology (1999) Fourth Edition byGrayson et al. (ed.) John Wiley & Sons, Inc., New York and inKirk-Othmer Encyclopedia of Chemical Technology Fourth Edition (1998 and2000) by Grayson et al. (ed.) Wiley Interscience (print edition)/JohnWiley & Sons, Inc. (e-format). Further relevant information can be foundin CRC Handbook of Chemistry and Physics (2003) 83rd edition by CRCPress. Details on conductive and other coatings, which can also beincorporated onto the nanowire surface by plasma methods and the likecan be found in H. S. Nalwa (ed.), Handbook of Organic ConductiveMolecules and Polymers, John Wiley & Sons 1997. See also, “ORGANICSPECIES THAT FACILITATE CHARGE TRANSFER TO/FROM NANOCRYSTALS,” U.S. Pat.No. 6,949,206. Details regarding organic chemistry, relevant for, e.g.,coupling of additional moieties to a functionalized surface can befound, e.g., in Greene (1981) Protective Groups in Organic Synthesis,John Wiley and Sons, New York, as well as in Schmidt (1996) OrganicChemistry Mosby, St Louis, Mo., and March's Advanced Organic ChemistryReactions, Mechanisms and Structure, Fifth Edition (2000) Smith andMarch, Wiley Interscience New York ISBN 0-471-58589-0, and U.S. PatentPublication No. 20050181195, published Aug. 18, 2005. Those of skill inthe art will be familiar with many other related references andtechniques amenable for functionalization of surfaces herein.

The polymer electrolyte coating may be directly linked to the surface ofthe nanowires, e.g., through silane groups, or may be coupled via linkerbinding groups or other appropriate chemical reactive groups toparticipate in linkage chemistries (derivitization) with linking agentssuch as, e.g., substituted silanes, diacetylenes, acrylates,acrylamides, vinyl, styryls, silicon oxide, boron oxide, phosphorusoxide, N-(3-aminopropyl)-3-mercapto-benzamide,3-aminopropyl-trimethoxysilane, 3-mercaptopropyl-trimethoxysilane,3-maleimidopropyl-trimethoxysilane, 3-hydrazidopropyl-trimethoxysilane,trichloro-perfluoro octyl silane, hydroxysuccinimides, maleimides,haloacetyls, hydrazines, ethyldiethylamino propylcarbodiimide, and/orthe like. Other surface functional chemistries can be used such as thosethat would be known to one or ordinary skill in the art.

In addition, a solubilized perfluorosulfonate ionomer (e.g., NAFION®)may be placed into the spare space between nanowires. The compositenanowire structure (e.g., as a porous sheet of interconnected nanowires,e.g., made by the process described in the Examples section), when notgrown in situ on one of the bipolar plates and/or proton exchangemembrane, may then be placed between bipolar plates on either side of aproton exchange membrane, and the assembly hot pressed to form acomplete membrane-electrode assembly fuel cell according to the presentinvention. The pressing temperature is determined such that the protonexchange membrane is softened in that temperature range, for example, to125° Celsius for NAFION®. The pressure level is about 200 kgf/cm₂. Inorder to efficiently distribute fuel/oxygen to the surface of theanode/cathode electrodes 808, 810, (see FIG. 8) a gas diffusion layer istypically needed in conventional fuel cells between the anode electrodeand bipolar plate on one side, and the cathode electrode and bipolarplate on the other side of the fuel cell. Typically, a carbon fibercloth is used as the gas diffusion layer. With the interconnectingnanowire composite membrane electrode catalyst support assembly of thepresent invention, this gas diffusion layer can be eliminated due to thesuperior structure of the nanowire-based electrodes.

In further embodiments, the present invention provides conductingcomposites, comprising one or more nanowires comprising a core, aninterfacial carbide layer and a carbon-based structure formed on theinterfacial carbide layer, and carbon black. As used throughout, carbonblack refers to the material produced by the incomplete combustion ofpetroleum products. Carbon black is a form of amorphous carbon that hasan extremely high surface area to volume ratio. The present inventionalso provides porous catalyst supports comprising one or more nanowirescomprising a core, an interfacial carbide layer and a carbon-basedstructure formed on the interfacial carbide layer, and carbon black.

As discussed throughout, suitably the carbon-based structures compriseat least one nanographitic plate formed on the interfacial carbidelayer. Exemplary cores for use in the conducting composites and porouscatalyst supports and comprise semiconductor material, such as Si, B,SiC or GaN.

In further embodiments, the core comprises an inorganic oxide selectedfrom the group consisting of SiO₂, Al₂O₃, TiO₂, SnO₂, ZrO₂, HfO₂ andTa₂O₅; an inorganic carbide selected from the group consisting of TiC,ZrC, HfC, NbC, WC, W₂C, MoC and Mo₂C; or an inorganic nitride selectedfrom the group consisting of TiN, ZrN, HfN, WN, MoN and BN. Examples ofinterfacial carbide layers include, but are not limited to, SiC, TiC,ZrC, HfC, NbC, WC, Mo2C and mixtures thereof. Suitably, the nanowiresfor use in the conducting composites comprise a core has across-sectional diameter of less than about 500 nm and a length ofgreater than about 50 nm.

As discussed throughout, suitably the nanowires comprise at least onenanographitic plate that extends away from the core a distance of about1 nm to about 100 nm, comprises at least 2-15 layers of graphene, and isoriented relative the major axis of the core at an angle of betweenabout 0° and about 90°.

Suitably, the porous catalyst supports of the present invention compriseone or more nanowires that are separated by a pore size of less thanabout 10 μm, for example, less than about 5 μm, e.g., less than about 1μm, e.g., less than about 0.2 μm, e.g., less than 0.02 μm, e.g., betweenabout 0.002 μm and 0.02 μm, e.g., between about 0.005 and 0.01 μm. Theoverall porosity of the porous catalyst supports may be greater thanabout 30%, for example, between about 30% and 95%, e.g., between about40% and 60%.

In a further embodiment, the present invention provides catalystscomprising one or more nanowires comprising a core, an interfacialcarbide layer and a carbon-based structure formed on the interfacialcarbide layer, carbon black and one or more catalyst metals associatedwith the nanowires and the carbon black (i.e., the nanowires and thecarbon black provide a composite support for the catalysts). Asdiscussed throughout, suitably the carbon-based structures comprise atleast one nanographitic plate formed on the interfacial carbide layer.Exemplary compositions and characteristics of nanowire cores,interfacial carbide layers, catalyst metals and nanographitic plates aredisclosed throughout and can be used in the catalysts of the presentinvention.

Micro Fuel cells Based on a Silicon Platform

In another embodiment, the present invention provides micro fuel cellsbased on a semiconductor wafer platform (e.g., Si) using the nanowires,bird's nest structures and interconnected nanowire networks describedthroughout. According to the embodiment, an integrated structurecomprising a bipolar plate composed of nanographite coated nanowireswith catalyst nanoparticles is prepared on an inorganic support wafer,e.g., a semiconductor wafer. For example, as shown in FIG. 22, aninorganic support wafer or semiconductor wafer 2202 (e.g., a highlydoped silicon wafer) is etched using a suitable etchant, for exampleNaOH, in step 2210, thereby producing one or more channels 2204 on afirst surface of the wafer. As used herein, the term channel means agrove, cut-out, recess, depression or other similar structure thatcreates a void in the material of the semiconductor wafer. Thesemiconductor wafer can be about 1 mm thick, for example a 1 mm thickdoped Si wafer. Other thicknesses and semiconductor wafer compositionscan also be utilized, as would become apparent to those of ordinaryskill in the art. The surfaces of the semiconductor wafer that areetched are generally those that have the greatest surface area, i.e. thefaces of the wafer. As shown in FIG. 22, channels 2204 are suitablyetched into the surfaces of the semiconductor wafer such that aserpentine, or other suitable formation, is produced. Any number (i.e.,1, 5, 10, 15, 20, etc.) or orientation of channels can be produced inthe wafer. Generally, channels 2204 are on the order of a few microns toa few millimeters wide and a few microns to a few millimeters deep.While channels 2204 are shown in FIG. 22 as uniform structures, they cantake any application-specific suitable shape or arrangement. By etchingboth surfaces of the semiconductor wafer 2202, i.e. the largest surfacesthat are opposite one another on the wafer, a structure is producedwhich comprises channels 2204 on opposite sides of the wafer 2202,thereby producing a precursor for a bipolar plate.

In step 2212, nanowires 2206 are then disposed in the channels 2204 leftby the etching process. Nanowires 2206 can be disposed in the channels2204 by either growing nanowires in the channels, or nanowires that havebeen grown externally from the channels can be harvested from a growthsubstrate and then deposited in the channels. Any suitable nanowiregrowth process, such as those described herein or known in the art, canbe used to grow the nanowires. For example, metallic catalystnanoparticles, e.g., Au nanoparticles, can be deposited in the channels2204, and then Si nanowires can be grown by using a VLS growth process,or other suitable process as described herein or known in the art.Suitably, nanowires 2206 will be disposed in the channels such that endsof the nanowires touch all faces of channels 2204 (i.e., the side(s) andbottom of the channels—if so configured). After nanowires 2206 aredisposed in the channels 2204, the surface of the semiconductor wafer2202 and the nanowires 2206 are contacted with a carbon-containing gasin step 2214 (carburization), thereby converting the surface of wafer2202 to silicon carbide 2208, e.g., SiC, and also coating the nanowireswith a carbide layer, e.g., silicon carbide. Methods for contacting withcarbon-comprising gases include those described herein, as well asothers known in the art. Further deposition of carbon on the surface ofthe nanowires results in the growth of graphene layers or sheets, asdescribed herein. In other embodiments, nanowires that have been grownexternally from the channels and harvested can first be carburized, andthen deposited into the channels formed in the semiconductor wafer.Generally, the wafer will also have been carburized prior to thenanowire deposition, though this can also occur after nanowiredeposition.

In step 2216, metal catalysts 2219, e.g., metal catalyst nanoparticles,are deposited on a surface of the nanowires 2206. In suitableembodiments, one surface of wafer 2202 comprises nanowires withdeposited anode nanoparticles (e.g., Pt), while the opposite surfacecomprises nanowires with deposited cathode nanoparticles (e.g., Pt—Ru).Step 2216 can also further comprise incorporation of electrolyte ontothe surface of the nanographite coated nanowires. The resultingstructure, therefore, is a bipolar wafer 2218 that can now be integratedinto MEAs and fuel cells.

In step 2222, bipolar wafer 2218 is combined with proton exchangemembranes to produce membrane electrode assemblies that can be used indirect methanol fuel cells. In one embodiment, the anode surface (i.e.,the surface of the wafer that comprises nanowires with anode catalysts)of bipolar wafer 2218 is disposed against a proton exchange membrane(e.g., a sulfonated tetrafluorethylene copolymer, such as NAFION®). Asecond bipolar wafer 2218 is then disposed against the membrane with itscathode surface (i.e., the surface of the wafer that comprises nanowireswith cathode catalysts) oppose the anode surface of the first wafer togenerate a membrane electrode assembly. This process can be repeated asmany times as required, disposing wafers and membranes against eachother to generate electrode assemblies. In further embodiments, plates(e.g., graphite, metal, semiconductor or other material) can be placedon the terminal ends of the assembly, thereby generating an enclosedassembly. The electrode assemblies described herein can then be used inmicro fuel cells, for example, direct methanol fuel cells 2109.

In another embodiment of the present invention, semiconductor wafer 2202(e.g., Si wafer) can be etched as described herein in step 2210 toproduce a series of channels 2204 in the wafer, suitably on oppositesides/surfaces of the wafer. The wafer can then be carburized in step2214′, thereby generating a carbide coating on the wafer (e.g., SiC). Instep 2220, these conducing carbide/graphite coated wafers (e.g., SiC)can then be combined with membrane electrode assemblies (MEA) that havebeen prepared by conventional methods to produce micro fuel cells, forexample direct methanol fuel cells 2209.

Additional methods for preparing fuel cell membrane electrode assembliesare also providing. An exemplary embodiment is represented in flow chart2300 in FIG. 23, and with reference to FIG. 24, showing a membraneelectrode assembly 2400 in accordance with one embodiment of the presentinvention. As shown in FIG. 23, in step 3202 a gas diffusion layer 2402is provided. Examples of materials suitable for use as gas diffusionlayers include, but are not limited to, TEFLON® (DuPont) treatedsurfaces, such as TEFLON® treated carbon paper or woven cloth (e.g.,carbon cloth). In step 2304, a first composition of catalystmetal-associated nanowires 2404 is disposed adjacent the gas diffusionlayer 2402. As used herein, the terms “disposed” and “disposed adjacent”are used to mean that the components are arranged next to each othersuch that the components are capable of interacting with one another soas to act as a membrane electrode assembly. Disposing componentsadjacent one another, includes, layering, applying, spraying, coating,spreading, or any other form of application of the various components.

In step 2306 of FIG. 23, a membrane layer 2406 is then disposed adjacentthe first catalyst metal-associated nanowire composition 2404. Suitably,membrane layer 2406 comprises a proton conducting polymer, such asNAFION® or other sulfonated polymer. Then, in step 2308 of FIG. 23, asecond composition of catalyst metal-associated nanowires 2408 isdisposed adjacent the membrane layer 2406.

In suitable embodiments, the first and second compositions of catalystmetal-associated nanowires (2404, 2408) comprise solutions ofcatalyst-associated nanowires, for example nanowire ink solutions. Thenanowire solutions of the present invention can also further compriseone or more additional components such as surfactants or polymers (forexample, to aid in nanowire dispersion) and/or ionomers, such asNAFION®. Suitably, the concentration of nanowires in the variousnanowires solutions are from about 0.01% to about 50% by volume, forexample, about 0.1% to about 20% by volume. Suitably, the first andsecond compositions of catalyst metal-associated nanowires are nanowiresolutions which also further comprise one or more ionomers, such asNAFION®.

Exemplary catalyst-associated nanowires for use in the methods of thepresent invention include those described throughout. Suitably, thefirst composition of catalyst metal-associated nanowires comprises asolution of anode catalyst metal-associated nanowires, and the secondcomposition of catalyst metal-associated nanowires comprises a solutionof cathode catalyst metal-associated nanowires, though the order of thetwo layers can be reversed. While any method of disposing the catalystmetal-associated nanowires can be used, suitably the nanowires aredisposing by spraying a solution of the nanowires onto the varioussurfaces. Methods for spraying nanowires onto various surfaces are wellknown in the art, see for example, U.S. Pat. No. 7,135,728, thedisclosure of which is incorporated herein by reference. Suitably thespraying methods utilize an ultrasonic bath to prevent aggregation ofthe nanowires in solution, and a computer-controlled spray nozzle todeliver the nanowire solution to the surfaces. In further embodiments,the spraying methods of the present invention comprise spraying multiplelayers of the nanowires (and one or more ionomers), so as to createmultiple layers of nanowires in the final MEA.

In additional embodiments, as shown in flowchart 2300 FIG. 23, themembrane electrode assembly preparation methods of the present inventionfurther comprise, in step 2310, disposing a masking layer (e.g.,metallic film or foil) adjacent the gas diffusion layer 2402 to cover atleast the edges of the gas diffusion layer prior to disposing of thefirst composition of catalyst metal-associated nanowires (e.g. nanowiresolutions comprising ionomers) 2404 in step 2304. A masking layer is notshown in FIG. 24, but suitably the masking layer is prepared such thatit covers the edges of gas diffusion layer 2402, but leaves an open,unmasked section in the center of the gas diffusion layer 2402. As shownin FIG. 24, disposing a first composition comprising catalystmetal-associated nanowires (e.g. a nanowire solution comprisingionomers) results in an assembly where the center of the gas diffusionlayer is covered by the nanowire composition, but the outer edges of thegas diffusion layer 2402 are not. Then, after disposing the firstcomposition, but prior to disposing the membrane layer 2406 in step2306, the masking layer is removed in step 2312. Thus, when the membranelayer 2406 is disposed adjacent the first catalyst metal-comprisingnanowire composition/layer 2404, the membrane layer 2406 not only ispositioned adjacent to the nanowires, but also the gas diffusion layer2402 at the edges.

In further embodiments, as shown in flowchart 2300 in FIG. 23, in step2514, a masking layer is disposed adjacent the membrane layer 2406 tocover at least the edges of the membrane layer prior to disposing of thesecond composition of catalyst metal-associated nanowires 2408 in step2308. This additional masking layer is not shown in FIG. 24, butsuitably the masking layer is prepared such that it covers the edges ofmembrane layer 2406, but leaves an open, unmasked section in the centerof the membrane layer 2406. As shown in FIG. 24, disposing a secondcomposition comprising catalyst metal-associated nanowires (e.g. ananowire solution comprising ionomers) 2408 results in an assembly wherethe center of the membrane layer 2406 is covered by the nanowires, butthe outer edges of the membrane layer 2406 are not.

Membrane electrode assemblies prepared by the methods of the presentinvention can be utilized in preparation of various fuel cellelectrodes, for example, in fuel cell electrode stacks. The methods ofthe present invention, suitably the nanowire spray methods, provide aquick and easy manufacturing process for preparing a four-layer membraneelectrode assembly.

In still further embodiments, the present invention provides methods forpreparing a fuel cell electrode stack 2500 utilizing the membraneelectrode assemblies disclosed throughout. In an exemplary embodiment,as shown in FIGS. 26A-26B, with reference to FIG. 25, flowchart 2600provides a method for preparing a fuel cell electrode stack. In step2602 of FIG. 26A, a first end plate 2502 is provided. As usedthroughout, end plates include machined graphite or molded conductingpolymer composites and other similar materials. In step 2604, a gasket2504 is disposed adjacent the end plate 2502. Suitably, gasket 2504 willcomprise a material that is able to create a seal between end plate 2502and additional components of the fuel cell stack. As shown in FIG. 25,gasket 2504 suitably has an opening to accommodate the addition of amembrane electrode assembly 2400. Exemplary materials for use as gasketsinclude, but are not limited to, various polymers and rubbers, such assilicon, MYLAR® (DuPont) laminates, ethylene propylene diene monomer(EPDM) rubber and the like.

In step 2606 of FIG. 26A, a membrane electrode assembly 2400 is disposedadjacent the gasket. While the four-layer membrane electrode assemblies(MEA) (2400) of the present invention are suitably used in the fuel cellelectrode stacks of the present invention, other membrane electrodeassemblies disclosed throughout and otherwise known in the art can alsobe used. Following disposition of the MEA, a gas diffusion layer 2506 isthen disposed adjacent the MEA in step 2608. In step 2610, an additionalgasket 2504 is then disposed adjacent gas diffusion layer 2506. Finally,in step 2612, an end plate 2502 is disposed adjacent gasket 2504 tocreate the completed fuel cell electrode stack 2500. The fuel cell stackcan then be clamped together for further processing. Addition of furthercomponents as disclosed throughout and known in the art can then beadded to yield a working fuel cell.

In additional embodiments, as shown in FIGS. 26A and 26B, the methods ofthe present invention can further comprise assembling additional MEAlayers (e.g., 2, 3, 4, 5, 6, etc., up to an n^(th) MEA) when preparingfuel cell electrode stacks 2500. That is, any number of MEA layers up toan n^(th), or final desired MEA layer, can be prepared in the fuel cellelectrode stacks. For example, following disposition of a second gasket2504 in step 2610, and prior to disposing a second end plate in step2612, a bipolar plate 2510 is disposed adjacent the second gasket 2504in step 2616. Both end plates 2502 and bipolar plate(s) 2510 suitablytypically have channels and/or grooves in their surfaces that distributefuel and oxidant to their respective catalyst electrodes, allow thewaste, e.g., water and CO₂ to get out, and may also contain conduits forheat transfer. Typically, bipolar plates and end plates are highlyelectrically conductive and can be made from graphite, metals,conductive polymers, and alloys and composites thereof. Materials suchas stainless steel, aluminum alloys, carbon and composites, with orwithout coatings, are good viable options for bipolar end plates in fuelcells. Bipolar plates and end plates can also be formed from compositematerials comprising highly-conductive or semiconducting nanowiresincorporated in the composite structure (e.g., metal, conductive polymeretc.). While bipolar plates suitably comprise channels and/or groves onboth surfaces, end plates typically only comprise channels and/or groveson the surface that is contact with the fuel cell components (i.e., theinternal surface), while the external surface does not comprise suchchannels or groves.

In step 2618 of FIG. 26B, an additional gasket 2504 is the disposedadjacent the bipolar plate 2510. In step 2620, a second MEA 2400 is thendisposed adjacent gasket 2504 (i.e. a third gasket as shown in FIG. 25).This is followed by disposing an additional gas diffusion layer 2606adjacent MEA 2400 in step 2622. In step 2624, an additional gasket 2504(i.e., fourth) is then disposed adjacent gas diffusion layer 2506.

As shown in FIG. 26B, in step 2626, steps 2616 through 2624 are thenrepeated until the n^(th), i.e., final, membrane electrode assembly(2400) has been disposed. By repeating the various steps of FIG. 26B,repeated layers of bipolar plate 2510, gasket 2504, MEA 2400, gasdiffusion layer 2506 and gasket 2504 are repeated until the desirednumber of MEAs (through the n^(th) MEA) are stacked together to form thefuel cell electrode stack. Once the final MEA has been utilized, ratherthan repeating steps 2616-2624 in step 2626, an end plate 2502 isdisposed adjacent the final gasket 2504 in step 2612 in FIG. 26A. Thepresent invention therefore provides methods for repeatedlydisposing/layering/stacking the various fuel cell components until thefinal, desired fuel cell stack is achieved.

The final fuel cell stack can then be clamped together, and fuelimpregnated with a suitable electrolyte, for example, an ethylene glycolsolution. Addition of further components as disclosed throughout andknown in the art can then be added to yield a working fuel cell.

Chromatographic Media

Particles prepared from the interconnected nanowire networks can be usedto create chromatographic media. For example, the particles can be usedin size exclusion columns by packing the mesoporous particulate materialin a column and then adding a solution containing appropriately-sizedlarge and small articles to be separated. Based upon the sizes of thepores, certain smaller articles will be retained by the particles (andthe column), while certain larger articles will pass through the packedcolumn. Such columns can be used for separation and sample cleanup, orin conjunction with analytic apparatus, e.g., chromatographs,fluorimeters and the like.

The interconnected nanowire networks are also useful as sample platformsor substrates for matrix-assisted laser desorption ionization (MALDI)time of flight Mass Spectrometry analysis of biomolecules, as well assurface coatings or scaffolds for other biological applications.

The interconnected nanowire networks are also useful as high surfacearea electrodes for medical devices, for example. In addition to highsurface area, the nanowire networks offer advantageous properties suchas long term stability, biocompatibility and low polarization overextended periods.

Field Emission Devices

Field emission devices are devices that capitalize on the movement ofelectrons. A typical field emission device includes at least a cathode,emitter tips, and an anode spaced from the cathode. (See, e.g., U.S.Pat. Nos. 7,009,331, 6,976,897 and 6,911,767; and U.S. PatentApplication No. 2006/0066217, the disclosures of each of which areincorporated herein by reference in their entireties). A voltage isapplied between the cathode and the anode causing electrons to beemitted from the emitter tips. The electrons travel in the directionfrom the cathode to the anode. These devices can be used in a variety ofapplications including, but not limited to, microwave vacuum tubedevices (e.g., X-ray tubes), power amplifiers, ion guns, high energyaccelerators, free electron lasers, and electron microscopes, and inparticular, flat panel displays. Flat panel displays can be used asreplacements for conventional cathode ray tubes. Thus, they haveapplication in television and computer monitors.

Conventional emitter tips are made of metal, such as molybdenum, or asemiconductor such as silicon. One of the concerns with using metalemitter tips is that the control voltage required for emission isrelatively high, e.g., around 100 V. Moreover, these emitter tips lackuniformity resulting in non-uniform current density between pixels.

More recently, carbon materials, have been used as emitter tips. Diamondhas negative or low electron affinity on its hydrogen-terminatedsurfaces. Carbon nanotubes, also known as carbon fibrils, have also beenexplored for use in emitter tip technology.

In another embodiment of the present invention, the nanowire structuresand interconnected nanowire networks described herein can be used asfield emitters and field emission devices. Exemplary field emissionelements in accordance with the present invention include, but are notlimited to, one or more vertically oriented refractory metal coatednanowires, one or more vertically oriented carbon coated nanowires(e.g., Si nanowires coated with SiC), a plurality of randomly orientednanowires covered with graphene layers (e.g., Si nanowires coated withSiC and graphene layers) and pieces of nanowires covered with graphenelayers, as well as combinations and variations thereof. As used herein,the term field emission element, field emitters and field emittingelements are used interchangeably to refer to structures that allowemission of electrons from their surface.

In suitable embodiments, the field emitters and field emission devicescomprise Si nanowires coated with SiC and graphene layers. As shown inFIG. 27, these “barbed” nanowires in a bird's nest configurationcomprise a structure much like a tetrapod, in that regardless of theorientation of the wire, carbon graphene layers extend outward andupward away from the nanowires. The thin graphene layers represent agood electron emitter, and the presence of several of these graphene“barbs” extending from all sides of a nanowire element ensure a highlevel of electron transmission.

By coating a substrate, for example a glass plate, with a plurality ofthe nanowire structures of the present invention, e.g., a bird's nestconfiguration, a highly uniform emitter element can be produced in whichSi nanowires coated with SiC and graphene layers form a series of highlyuniform, extended emission elements.

In another embodiment, refractory metal coated nanowires can also serveas emitting elements. For example, Mo coated nanowires (e.g., Sinanowires). Other metallic coatings can also be applied to the nanowiresto allow them to function as emitting elements.

It will be readily apparent to one of ordinary skill in the relevantarts that other suitable modifications and adaptations to the methodsand applications described herein can be made without departing from thescope of the invention or any embodiment thereof. Having now describedthe present invention in detail, the same will be more clearlyunderstood by reference to the following examples, which are includedherewith for purposes of illustration only and are not intended to belimiting of the invention.

EXAMPLES Example 1 Preparation of Nanowire Structures

Silicon nanowires coated with WO₃ were heated at 650° C. for 30 minutes,followed by 1250° C. for 6 min in the presence of a flowing gas mixturecomprising Ar (430 cc/min), H₂ (130 cc/min) and CH₄ in Ar (228 cc/min).After the preparation was cooled, an interconnected nanowire network 300comprising interconnected nanowire structures 100 was formed. Thenanowire structures 100 comprising Si nanowire cores 102, carbon basedlayers 104 (SiC/WC interfacial carbide layers) and carbon-basedstructures 106 (graphene nanographitic plates) connecting the nanowirestructures 100.

Example 2 Deposition of Nanoparticles on Nanowires

Approximately 10 mg Si nanowires were dispersed in ethanol by sonicationto form a nanowire suspension. An interconnected nanowire network wasprepared by vacuum filtration of the nanowire suspension over apolyvinylidene fluoride (PVDF) membrane and vacuum drying, then 2 cc0.1% polylysine solution was added to the filter funnel to absorbpolylysine on the surface of the nanowires. After 5 minutes, all liquidin the funnel was vacuum removed and the nanowire network was separatedfrom the PVDF membrane. After being dried in an oven at 100° Celsius for15 minutes, the nanowire network was submerged in 10 cc of Au colloidsolution (10 nm colloids) and soaked for 20 minutes to absorb the Aunanoparticles on the surface of the nanowires. Finally, the nanowirenetwork was removed from the Au colloid solution, rinsed with isopropylalcohol (IPA), and dried at 100° Celsius to obtain a nanowire networkcoated with gold nanoparticles. FIG. 14 shows the SEM image of the Aucatalyst nanoparticles deposited on the network of interconnectednanowires.

Example 3 Fabrication of Densely Packed Nanowires

In order to examine methods to increase packing density of nanowires,“bird's-nest” structures of 60 nm silicon nanowires were prepared usingvarious techniques. The results of the methods are represented below inTable 1:

TABLE 1 Packing Density Relative Method to Silicon Density As collectedon the filter   6% Stirred with shear mixer  6.1% Uniaxially pressed11.2% Surfactant (Triton X-100)   8% Die pressed, 40 KPSI 46.6% CompactPacking (90°) 78.5% Compact Packing (0°) 90.7%

The packing density prepared by filtering the dispersion of 60 nmnanowires was 6%. Stirring the dispersion with a high speed shear mixerprior to filtration did not improve the packing density. A packingdensity increase of more than 30% was noted by adding surfactant (TritonX-100) to the nanowire dispersion prior to filtration. A significantincrease in packing density was observed after application of uniaxialpressure to the nanowires. A packing density of 46.6% was measured froma thin nanowire pellet prepared by die pressing nanowires at 40 KPSIafter grinding in a pellet die. Scanning electron microscopy indicatedthat nanowires were short in this sample, a result of grinding and/orpressing.

Example 4 Fuel Cell Performance Characterization

Results of various performance characterization experiments arepresented herein. These results indicate the significant performanceimprovements created by the use of the nanowire networks of the presentinvention (i.e., Bird's Nest architecture) both interconnected andnot-interconnected in fuel cell applications such as direct methanolfuel cells.

An almost 3-fold increase in methanol oxidation activity relative tocommercial carbon black catalysts through a combination of a newcatalyst particle material and the unique structural characteristics ofthe Bird's Nest architecture has been observed. In addition, a 51%increase in kinetic current for O₂ reduction compared to carbon blackhas been measured, indicating a strong influence of support interactionson catalytic efficiency from our unique highly-crystalline nanographitesurface coating on the Bird's Nest structure. A 2.5× increase indiffusion efficiency for methanol through the birds nest structurecompared to carbon black has been measured, which should allow up to2.5× higher loading of NAFION® in the catalyst layer without impactingdiffusion efficiency; this has the potential to increase protonconductivity in Bird's Nest membrane electrode assembly by up to 2.5×relative to carbon black.

In order to determine the accessibility/diffusion characteristics ofnanowire structures of the present invention, oxygen reduction activitywas determined. FIG. 28 shows a plot of current vs. potential comparinga Pt rotating disc electrode (Pt RDE), a carbon layer on the Pt-RDE(C/Pt RDE), and a carbonized Si nanowire layer (i.e., SiC nanowires)placed on the Pt-RDE (NW/Pt RDE). The SiC nanowire layer providesincreased access to the Pt electrode (proton accessibility of 86%compared to Pt-RDE), as compared to a carbon layer (proton accessibilityof 46% compared to Pt-RDE), resulting in current reduction of only 48%relative to 61% compared to the carbon layer at a potential of 0.3V asshown in FIG. 28.

FIG. 29 shows a representative performance curve for methanol oxidationcomparing the current commercial catalyst material on carbon black(manufactured by ElectroChem. Inc., Woburn, Mass.), and the catalystmaterial of the present invention on a carbon black support. This dataindicates an increased catalytic activity of approximately 65% over,resulting from the new catalyst material alone.

In addition to the inherent increase in catalytic efficiency of thecatalyst particles of the present invention, improved catalytic activityresulting from the interaction of these particles with the uniquesurface of the nanographite nanowires has also been observed.Kotecky-Levich analysis for oxygen reduction (FIG. 30) on a nanowireBird's Nest structure in an electrochemical half-cell showed a 51%increase in kinetic current at 0.6V in the Bird's Nest structure overcarbon black made with the same catalyst particles. This increase inreaction rate can be explained by an increase in catalytic efficiencyresulting from interaction with the nanographite support material.

Measurements of the 3-phase contact surface area for the Bird's NestMEAs of the present invention show a 3-phase contact density of 676cm²/mg compared to only 432 cm²/mg for commercially available carbonblack MEAs (ElectroChem. Inc.). This represents a 56% increase in3-phase contact density in these preliminary Bird's Nest structures.Direct measurement of 3-phase contact showed a total of 758 cm²/mg forthe carbon black MEA, a 75% increase relative to commercial carbonblack. Correcting for the difference in average particle size betweenthe two catalyst particles, on average, 21% more catalyst particles areavailable for 3-phase contact from the carbon black using the catalystparticles of the present invention over commercial carbon blackcatalysts formed by “impregnation” methods.

FIG. 31 shows methanol oxidation activity in an electrochemicalhalf-cell over a period of 20 minutes for a Bird's Nest nanowirecatalyst, a commercial carbon black catalyst and a carbon black catalystformed using the PtRu anode catalyst material of the present invention.This plot clearly shows an improvement in stability over this timeperiod for the Bird's Nest structure, even compared to the identicalcatalyst material on carbon black. This appears to be evidence of apurely structural effect on stability, even over time periods as shortas 20 minutes.

In contrast to the porous nature of packed particle carbon black, theintrinsically open, non-tortuous network of nanowires in a Bird's Neststructure according to the present invention allows for efficientdiffusion of fuel and waste into and away from the catalytic sites.Theoretical modeling of the diffusion characteristics of a bird's neststructure indicates that diffusion of solutions through this type of anetwork is approximately the same as diffusion in bulk solution. Asevidence of this effect, significantly increased catalytic activity formethanol oxidation in electrochemical half-cell measurements betweenBird's Nest and carbon black support structures using the same catalystmaterials have been observed.

FIG. 32 and Table 2 below demonstrate the relative activity of these twostructures in addition to commercial carbon black catalyst, displayingthe purely structural impact of the Bird's Nest architecture on improvedactivity. Overall, the current Bird's Nest catalyst layer has almost 3×higher activity for methanol oxidation compared to current commercialcarbon black catalyst in these half-cell measurements. This is theresult of a combination of increased catalytic activity and diffusioneffects.

TABLE 2 % increase in activity over Commercially- Current at 0.6 Vavailable Commercially available carbon 0.6 mA   1x black catalystCarbon black with Catalyst of the   1 mA 1.67x present inventionNanowire Bird's Nest with 1.7 mA 2.83x Catalyst of the present invention

Results of the methanol oxidation activity of PtRu anode catalysts arepresented in Table 3 below.

TABLE 3 Relative Activity (%) Hydrogen initial^(b) 20 min^(c)Sample/Electrode area (%)^(a) 25° C. 40° C. 25° C. 40° C. Commercial 100100 100 100 100 carbon Catalysts of the 110 138 164  97 167 presentinvention on Carbon Catalysts of the 118 274 283 204 278 presentinvention on Nanowire ^(a)current integration of the cyclic voltammogramin the hydrogen oxidation region; ^(b)based on linear potential scandata at 0.6 V; ^(c)based on data recorded after 20 min of constantpolarization at 0.6 V.

Oxygen reduction activity was also measured using a commerciallyavailable platinum catalyst on a carbon support (manufactured byElectroChem. Inc., Woburn, Mass.), platinum catalysts of the presentinvention on a carbon support, and platinum catalysts of the presentinvention on SiC nanowires. FIG. 33 shows a plot of current vs.potential comparing these three Pt catalyst supports. Results of theanalysis, summarized below in Table 4, indicate a higher current for thenanowire-supported anode Pt catalyst, resulting in an 18% improvementover the conventional carbon supports.

TABLE 4 Relative Oxygen Reduction Activity Hydrogen (%) Area TotalKinetic Diffusion Sample/Electrode (%)^(a) Current^(b) Current^(c)Current^(d) Commercial 100 100 100 100 carbon Catalysts of the 168 96100 100 present invention on Carbon Catalysts of the 168 114 151 118present invention on Nanowire ^(a)current integration of the cyclicvoltammogram in the hydrogen oxidation region; ^(b)linear potential scanin oxygen saturated solution at 3600 RPM, data at 0.6 V, ^(c)analysisbased on Koutecky-Levich equation, compared at 0.6 V; ^(d)analysis basedon Koutecky-Levich equation, assuming identical number of electronstransferred for all catalysts

Direct measurements of the diffusion efficiency of O₂ through Bird'sNest nanowire and carbon black structures determined the averageeffective pore length for Bird's Nest supports to be 1.1 nm, while forcarbon black, it is 2.8 nm, indicating a 2.5× increase in diffusionefficiency through the Bird's Nest structure due to the open,non-tortuous diffusion pathway that is inherent to this structure.

Example 5 Dispersion Improvement of Nanowire Electrodes

Platinum catalyst loading, NAFION® coverage and fuel cell performancewere characterized for electrodes comprising: 1) ECCMEA#1: commerciallyavailable catalyst nanoparticles and carbon support (manufactured byElectroChem, Inc., Woburn, Mass.); 2) NSCMEA#1: platinum catalystprepared according to the present invention disposed on carbon support;and 3) NWMEA#13 and #16: two samples of platinum catalyst preparedaccording to the present invention disposed on SiC nanowires. Pt loadingand NAFION® coverage results are presented below in Table 5:

TABLE 5 Pt NAFION ® Pt area/Pt Loading Coverage (mg- Pt Area weight(mg/cm²) NAFION ®/m²) (cm²-Pt) (cm²/mg) ECCMEA#1 0.256 4.56 572 431NSCMEA#1 0.310 4.76 1175 758 NWMEA#13 0.307 6.11 6662 432 NWMEA#16 0.3004.93 446 298

FIGS. 34A and 34B represent performance of the electrodes describedabove showing IR corrected potential versus current density, andaccessibility loss (fitting at 3-5 mA/cm²) respectively of the ECCMEA#1,NWMEA#13 and NWMEA#16 electrodes. It should be noted that fuel cellperformance, as well as accessibility loss, for the two nanowiresupported electrodes are virtually identical.

FIGS. 35 and 36 of Nanosys presentation represent results of the O₂partial pressure variation for an electrode prepared from Pt catalyticnanoparticles of the present invention disposed on a carbon support(NSCMEA#2) and Pt catalytic nanoparticles of the present inventiondisposed on SiC nanowires of the present invention (NWMEA#16). Theresults are also represented in the table below. FIG. 35 represents IRcorrected potential characteristics of the electrodes at two differentO₂ pressures (20% and 5%). FIG. 36 represents the potential variation(V) for the same two samples over a range of current densities. Theresults of these measurements, as noted below in Table 6, indicate thatO₂ diffusivity using the nanowire support appears to be equal to, oreven better than, the traditional carbon support.

TABLE 6 NAFION ® Pt area/Pt Pt loading coverage (mg- weight (mg/cm²)NAFION ®/m²) (cm²/mg) NSCMEA#2 Pt/C 0.361 4.56 477 NWMEA#16 Pt/nanowire0.3 4.93 298

Example 6 Characterization of Bird's Nest Electrodes

The following procedure was followed to generate a “bird's nest”nanowire support with Pt catalyst nanoparticles. Si nanowires preparedaccording the present invention were deposited in a random fashion, soas to form a “Bird's Nest” structure of overlapping nanowires. Thenanowire structure was then contacted with a carbon-comprising gas(e.g., CH₄) in order to form a carbide layer on the surface of thenanowires, followed by formation of graphene sheets on the surface ofthe SiC nanowires. Pt catalyst nanoparticles of the present inventionwere then deposited onto the graphite coated SiC nanowires. Finally, thecatalyst nanoparticle/SiC nanowire bird's nest structure wasimpregnated, or hand painted, with the proton conductor, NAFION®. Theresulting electrode was characterized with approximately 11% (wt %) Ptloading onto the SiC nanowires, and approximately 0.15 mg-Pt/cm² ofelectrode.

FIG. 37 shows the IR corrected potential versus current densitymeasurements for four electrodes. ECCMEA#1: Electrochem® Pt catalyst oncarbon substrate; NWMEA#13: Pt catalyst prepared according to thepresent invention on SiC nanowire bird's nest hand painted with NAFION®;NWMEA#14 and #15: Pt catalyst prepared according to the presentinvention on SiC nanowire bird's nest impregnated with NAFION®. Theresults of the characterization are also presented in Table 7 below(NSCMEA#1 represents Pt catalyst prepared according to the presentinvention on carbon substrate):

TABLE 7 NAFION ® Pt Coverage Pt Loading (mg- area Pt area/Pt (mg/NAFION ®/ (cm²- weight cm²) m²) Pt) (cm²/mg) ECCMEA#1 Pt/C 0.265 4.56572 431 (Electrochem) NSCMEA#1 Pt/C 0.31 4.76 1175 758 NWMEA#13 HandPaint 0.307 6.11 662 432 NAFION ® NWMEA#14 NAFION ® 0.123 N/A 416 676Impregnation NWMEA#15 NAFION ® 0.156 1.94 52 67 Impregnation

Example 7 Refluxing to Prepare Catalyst Metal-associated Nanowires

290 mg of the nanowire structures of the present invention are dispersedin 20 mL of ethylene glycol. To the dispersion, 51 mL of Pt—Ru colloidsolution (3 mg metal/mL) are then added. This solution is then refluxedfor 60 minutes.

The solution is then cooled to room temperature and 2M nitric acid isadded drop-wise until the solution attains a pH of about 4.0. Thissolution is then stirred at room temperature for about 4 hours, afterwhich 2M nitric acid is added drop-wise until the solution attains a pHof about 1.0. The solution is then stirred for about 1 hour.

The solution is then filtered, and washed three times with deionizedwater. The resulting nanowire cake is then dispersed in deionized water,filtered, and then washed a gain. The resulting nanowire cake is thendried at 120° C. under vacuum overnight. The resultingcatalyst-associated nanowires can then be dispersed in a solvent forapplication to various substrates or other uses, or utilized in theirdried state. FIG. 38 shows transmission electron micrographs (twomagnifications) of catalyst metal-associated nanowires preparedaccording to the methods of the present invention.

Exemplary embodiments of the present invention have been presented. Theinvention is not limited to these examples. These examples are presentedherein for purposes of illustration, and not limitation. Alternatives(including equivalents, extensions, variations, deviations, etc., ofthose described herein) will be apparent to persons skilled in therelevant art(s) based on the teachings contained herein. Suchalternatives fall within the scope and spirit of the invention.

All publications, patents and patent applications mentioned in thisspecification are indicative of the level of skill of those skilled inthe art to which this invention pertains, and are herein incorporated byreference to the same extent as if each individual publication, patentor patent application was specifically and individually indicated to beincorporated by reference.

1. A nanowire comprising a carbon-based layer, wherein the carbon-basedlayer is substantially devoid of basal plane carbon.
 2. The nanowire ofclaim 1, wherein the carbon-based layer contains less than about 0.5%basal plane carbon.
 3. The nanowire of claim 1, further comprising acore.
 4. The nanowire of claim 3, wherein the core comprises asemiconductor material.
 5. The nanowire of claim 4, wherein thesemiconductor material is selected from the group consisting of Si, B,SiC and GaN.
 6. The nanowire of claim 4, wherein the semiconductormaterial is doped.
 7. The nanowire of claim 3, wherein the core and/orthe carbon-based layer comprise carbon.
 8. The nanowire of claim 3,wherein the core and/or the carbon-based layer consist essentially ofcarbon.
 9. The nanowire of claim 3, wherein the core and/or thecarbon-based layer consist of carbon.
 10. The nanowire of claim 3,wherein the core comprises carbide.
 11. The nanowire of claim 10,wherein the core comprises SiC.
 12. The nanowire of claim 3, wherein thecore is Si and the carbon-based layer is SiC.
 13. The nanowire of claim3, wherein the core is SiC and the carbon-based layer is carbon.
 14. Thenanowire of claim 1, wherein the carbon-based layer is between about 1nm to about 500 nm in thickness.
 15. A method of manufacturing ananowire, comprising: (a) heating a nanowire core; and (b) contactingthe nanowire core with one or more carbon-comprising gases to form acarbon-based layer on the nanowire core, wherein the carbon-based layeris substantially devoid of basal plane carbon.
 16. The method of claim15, wherein said heating is to a temperature of greater than about 600°C.
 17. The method of claim 15, wherein said contacting step comprisescontacting with a gas comprising carbon monoxide, methane, ethane,propane, butane, ethylene or propylene and optionally further comprisescontacting with a gas comprising He, Ne, Ar, Kr, Xe, or H₂.
 18. A methodof manufacturing a nanowire, comprising: (a) heating a nanowire core;(b) contacting the nanowire core with one or more carbon-comprisinggases to faun a carbon-based layer on the nanowire core; and (c) forminga precursor coating selected from the group consisting of TiO₂, ZrO₂,HfO₂, Nb₂O₃, Ta₂O₅, MoO₃ and WO₃ on the nanowire core prior to theheating step.
 19. A method for preparing a fuel cell electrodecomprising: providing a semiconductor wafer having a first surface and asecond surface; forming one or more channels on the first surface andthe second surface; disposing one or more nanowires in the channels inthe first and second surfaces; contacting the nanowires and the firstand second surfaces with one or more carbon-comprising gases to form acarbon-based layer on the nanowires and the first and second surfaces;and disposing one or more metal catalysts on the nanowires.
 20. Themethod of claim 19, wherein said forming of one or more channelscomprises etching.
 21. The method of claim 20, wherein said etchingcomprises etching with NaOH.
 22. The method of claim 19, wherein saiddisposing of one or more nanowires comprises growing nanowires in thechannels.
 23. The method of claim 19, wherein the disposing of one ormore metal catalysts on the nanowires comprises depositing one or morePt nanoparticles on nanowires on the first surface and PtRunanoparticles on nanowires on the second surface.
 24. A fuel cellelectrode prepared by the method of claim 19.